Recently a user revived an old thread by @20-100 concerning mail clients.

Today we discuss how to setup neomutt as a comfortable, integrated and versatile CLI MUA, while trying to keep resource requirements as low as possible, so as to ideally provide a viable solution for legacy hardware, embedded systems limited in specs and headless / console-only computers.

requirements

mail/neomutt (you may also use mutt, but some features will be unsupported unless adding local patches)
mail/msmtp, as SMTP client
security/gnupg2, to encrypt/decrypt and sign messages, as well as encrypt/decrypt passwords on the fly. This tutorial assumes you already configured GnuPG properly to use your secret key(s).
databases/abook as an addressbook to store and manage aliases
a third-party editor, if you find nvi (or whatever $EDITOR is set to) to be too conservative

preliminary tasks

Create directories to store mutt files

$ for d in accounts bodies cache certificates headers private themes tmp
    > do mkdir -p ~/.mutt/${d}
    > done

store your imap/smtp password as an encrypted hidden file

$ echo -e "your password here\n" | gpg2 --encrypt -o ~/.msmtp.gpg # enter key ID
$ chmod 600 ~/.msmtp.gpg

generate self-signed client certificate and for mutt:

$ openssl req -x509 -newkey rsa:4096 -days 365 -keyout ~/.mutt/private/mutt.key -out ~/.mutt/certificates/mutt.crt

create a ~/.signature file with your signature, try not to make it longer than 4 lines:

Real Name ~ xmpp:buddy@jabber.sdf.org
SDF Public Access UNIX System - https://sdf.org
PGP fingerprint = XXXX XXXX XXXX XXXX

configure msmtp by editing a ~/.msmtprc file like below:

# Set default values for all following accounts.
defaults
auth           on
tls            on
tls_starttls   on
tls_trust_file /etc/openssl/certs/ca-certificates.crt
tls_cert_file  ~/.mutt/certificates/mutt.crt
tls_key_file   ~/.mutt/private/mutt.key
syslog         on
tls_certcheck  on
logfile        ~/.msmtp.log

# RiseUp 
account        riseup
host           mail.riseup.net
port           587
user           account@riseup.net
from           your real name <account@riseup.net>
passwordeval   "gpg2 --quiet --for-your-eyes-only --no-tty --decrypt ~/.msmtp.gpg"

# Outlook 
account        outlook
host           smtp.office365.com
port           587
user           account@outlook.it
from           your real name <account@outlook.it>
passwordeval   "gpg2 --quiet --for-your-eyes-only --no-tty --decrypt ~/.outlook.gpg"


account  default : riseup

configuration files

DISCLAIMER: What follows is a sample configuration, modeled after my personal one. Feel free to experiment with it a bit and adjust it to your needs.
Please also notice this setup uses Riseup.net as a hypothetical mail privider.

Change working directory to ~/.mutt and touch/edit the required files, as shown below:

muttrc

# =================== MUTT CONFIGURATION ==================== #
## S S L
# force encryption
set ssl_force_tls	= "yes"
# default to TLSSv1.3 
set ssl_use_tlsv1_3	= "yes"
# and STARTTLS
set ssl_starttls	= "yes"

## e n c o d i n g
# default to unicode (using iso-8859-1 as fallback)
set send_charset	= "us-ascii:utf-8:iso-8859-1"
# decode RFC-2047-encoded MIME parameters
set rfc2047_parameters  = "yes"
# decode headers/bodies before searching
set thorough_search	= "yes"

## a p p e a r a n c e
set date_format		= "%y/%m/%d %I:%M%p"
set index_format	= "%2C %zs %?X?A& ? %D %-15.15F %s (%-4.4c)"
set pager_format	= "-%Z- %C/%m: %-20.20n   %s%*  -- (%P)"
set pager_context	= "3"
set forward_format 	= "Fwd: %s"
# Disables the `+` displayed at line wraps
set markers           = "no"

## h e a d e r s
ignore "Authentication-Results:"
ignore "DomainKey-Sighnature:"
ignore "DKIM-Signature:"
hdr_order From: Resent-From: Reply-To: X-Mailer: User-Agent: Date: To: Cc: Subject:
my_hdr Organization: Riseup Networks Collective
my_hdr PGP: s
my_hdr User-Agent: `/usr/pkg/bin/neomutt -v | head -n 1`
my_hdr X-Operating-System: `uname -s`, kernel `uname -r`

## d i r e c t o r i e s
set header_cache	= "~/.mutt/cache/headers"
set message_cachedir	= "~/.mutt/cache/bodies"
set certificate_file	= "~/.mutt/certificates"
set tmpdir		= "~/.mutt/tmp"

## a l i a s e s
set alias_file 		= "~/.mutt/aliases"
# sort aliases alphabetically
set sort_alias 		= alias
# display real names first
set reverse_alias 	= yes
# external address book command
set query_command	= "abook --mutt-query '%s'"


## m i s c 
# supress waiting for informational messages
set sleep_time		= "0"             
# lock session after 15 min
set timeout		= "900"
# display plain text bodies whenever possible
alternative_order text/plain text/enriched text/html
# most recent messages displayed first
set sort 		= "reverse-date"
# external editor
set editor		= "/usr/pkg/bin/vim"
# do not automatically jump to next message
set pager_stop 		= "yes"              
# Unread mail stay unread until read
set mark_old 		= "no"              
# attachments are forwarded with mail
set mime_forward 	= "yes"         
# mutt won't ask "press key to continue"
set wait_key 		= "no"              
# skip to compose when replying
set fast_reply   	= "yes"              
# save attachments with the body
set fcc_attach    	= "yes"             
# include message in forwards
set forward_quote       = "yes"
# reply as whomever it was to
set reverse_name        = "yes"       
# include message in replies
set include             = "yes"      
# avoid lags using IMAP with some email providers
set mail_check		= "60"              
# automatically show html (mailcap uses w3m)
auto_view text/html
# and encrypted messages            
auto_view application/pgp-encrypted
# notifications
set new_mail_command	= "notify-send -i /usr/pkg/share/icons/Adwaita/16x16/status/mail-unread.png -a "mutt" 'New Email in %f' '%n new messages, %u unread.' &"



# external configuration files to read
source ~/.mutt/accounts/imap.riseup
source ~/.mutt/sidebar
source ~/.mutt/binds
source ~/.mutt/gpg.rc
source ~/.mutt/themes/custom
source ~/.mutt/aliases

accounts/imap.riseup

# =================== ACCOUNT SETTINGS==================== #
# general settings
set realname	   = "your real name here"
set imap_user	   = "account@riseup.net"
set imap_pass	   = "`gpg2 --quiet --for-your-eyes-only --no-tty --decrypt ~/.msmtp.gpg`"
set folder	   = "imaps://mail.riseup.net:993/"
set spoolfile	   = +INBOX
set postponed	   = +Drafts
set record 	   = +Sent
set trash 	   = +Trash
set sendmail	   = "/usr/pkg/bin/msmtp -a riseup -t"
set from	   = "your real name <account@riseup.net>"
set signature	   = "~/.signature"
set mail_check 	   = 90
# generate 'From:' headers
set use_from   = yes
# fetch the set of subscribed folders from server
set imap_check_subscribed = "yes"
# only display subscribed folders
set imap_list_subscribed  = "yes"
# make use of the IMAP IDLE extension
set imap_idle		  = "yes"
# poll open IMAP conections every 3 mins
set imap_keepalive 	  = 180

gpg.rc

# =================== PGP SETTINGS ==================== #

# g e n e r a l
set crypt_autosign	= "yes"
set pgp_sign_as		= "account@riseup.net"
set pgp_sign_as		= "your PGP key ID"
# no mail encryption by default
unset crypt_autoencrypt
# but encrypt replies to encrypted mails 
set crypt_replyencrypt	= "yes"
# always attempt to verify signatures
set pgp_verify_sig	= "yes"
# encode signed messages as quoted-printable
set pgp_strict_enc	= "yes"
# let gpg-agent handle passphrase prompts
set pgp_use_gpg_agent=yes
# cached passphrase shall expire after 1h time
set pgp_timeout = 3600

# m a c r o s
set pgp_decode_command       = "gpg2 %?p?--passphrase-fd 0 --pinentry-mode=loopback?  --no-verbose --batch --output - %f"
set pgp_verify_command       = "gpg2 --no-verbose --batch --output - --verify %s %f"
set pgp_decrypt_command      = "gpg2 %?p?--passphrase-fd 0 --pinentry-mode=loopback? --no-verbose --batch --output - %f"rsa
set pgp_sign_command         = "gpg2 --no-verbose --batch --output - %?p?--passphrase-fd 0 --pinentry-mode=loopback? --armor --detach-sign --textmode %?a?-u %a? %f"
set pgp_clearsign_command    = "gpg2 --no-verbose --batch --output - %?p?--passphrase-fd 0 --pinentry-mode=loopback? --armor --textmode --clearsign %?a?-u %a? %f"
set pgp_encrypt_only_command = "/usr/pkg/libexec/neomutt/pgpewrap gpg2 --batch --quiet --no-verbose --output - --encrypt --textmode --armor --always-trust -- -r %r -- %f"
set pgp_encrypt_sign_command = "/usr/pkg/libexec/neomutt/pgpewrap gpg2 %?p?--passphrase-fd 0 --pinentry-mode=loopback? --batch --quiet --no-verbose --textmode --output - --encrypt --sign %?a?-u %a? --armor --always-trust -- -r %r -- %f"
set pgp_import_command       = "gpg2 --no-verbose --import -v %f"
set pgp_export_command       = "gpg2 --no-verbose --export --armor %r"
set pgp_verify_key_command   = "gpg2 --no-verbose --batch --fingerprint --check-sigs %r"
set pgp_list_pubring_command = "gpg2 --no-verbose --batch --quiet --with-colons --with-fingerprint --list-keys %r"
set pgp_list_secring_command = "gpg2 --no-verbose --batch --with-colons --with-fingerprint --list-secret-keys %r"

binds

# =================== SHORTCUTS ==================== #
# Non-standard, more vim-like key mappings
#
# Free keys
bind index h noop
bind index,pager d noop #used for dX
bind index,pager i noop #used for goto iXY and i[1-9] in account muttrc (XY = 2 mailbox letters)
bind index,pager M noop #used for CXY, "move" to XY in account muttrc
bind index,pager C noop #used for CXY, "copy" to XY in account muttrc
bind pager,attach,browser,index g noop

# i n d e x
bind index gg first-entry
bind index G last-entry
bind index p recall-message
bind index R group-reply
bind index <tab> sync-mailbox
bind index <space> collapse-thread
macro index,pager H view-mailcap
bind index l display-message
bind browser l select-entry
bind index gl limit
macro index gL "<limit>all\n" "show all messages (undo limit)"
bind index \031 previous-undeleted # Mouse wheel
bind index \005 next-undeleted # Mouse wheel
bind browser,pager,index        N   search-opposite
bind pager,index                dT  delete-thread
bind pager,index                dt  delete-subthread
bind pager,index                gt  next-thread
bind pager,index                gT  previous-thread
bind index                      za  collapse-thread
bind index                      zA  collapse-all
bind index                      -   collapse-thread
bind index                      _   collapse-all
macro index \Cr "<tag-pattern>~U<enter>\
  <tag-prefix><clear-flag>N<untag-pattern>.<enter>" \
  "mark all messages as read"

# m i s c
bind browser,pager,index        N   search-opposite
bind pager,index                dT  delete-thread
bind pager,index                dt  delete-subthread
bind pager,index                gt  next-thread
bind pager,index                gT  previous-thread
bind index,pager V view-raw-message 
macro index dd "<save-message>+Trash<enter><sync-mailbox>" "move message to Trash"
bind pager,index D  purge-message
bind index,pager gr group-reply #R is recall postponed by mutt

# p a g e r
bind pager  k previous-line
bind pager  j next-line
bind pager l view-attachments
bind pager  R group-reply
bind pager gg top
bind pager G bottom
bind pager,attach h exit
bind pager \031 previous-line # Mouse wheel
bind pager \005 next-line # Mouse wheel
bind pager t display-toggle-weed

# b r o w s e r
bind browser gg top-page
bind browser G bottom-page
bind browser l select-entry

### a t t a c h m e n t s
bind attach l view-text
bind attach <return> view-mailcap
bind attach gg first-entry
bind attach G last-entry
bind attach l view-text
bind attach <return> view-mailcap

# e d i t o r 
bind editor <space> noop
bind editor <Tab> complete-query

# a d d r e s s  b o o k
macro index,pager  a "<pipe-message>abook --add-email-quiet<return>" "Add this sender to Abook"

# p g p
bind compose p pgp-menu
macro compose Y pfy "send mail without GPG"

# m a i l b o x e s
macro index <f4> '<sync-mailbox><enter-command>source ~/.mutt/accounts/mdir.riseup<enter><change-folder>!<enter>'
macro index,pager c "<change-folder>?<toggle-mailboxes>" "open a different folder" 

macro index,pager iqu "<change-folder>=Queue<enter>" "go to Queue"
macro index,pager Mqu "<save-message>=Queue<enter>" "move mail to Queue"
macro index,pager Cqu "<copy-message>=Queue<enter>" "copy mail to Queue"

macro index,pager itr "<change-folder>=Trash<enter>" "go to Trash"
macro index,pager Mtr "<save-message>=Trash<enter>" "move mail to Trash"
macro index,pager Ctr "<copy-message>=Trash<enter>" "copy mail to Trash"

macro index,pager isp "<change-folder>=Spam<enter>" "go to Spam"
macro index,pager Msp "<save-message>=Spam<enter>" "move mail to Spam"
macro index,pager Csp "<copy-message>=Spam<enter>" "copy mail to Spam"

macro index,pager iin "<change-folder>=INBOX<enter>" "go to INBOX"
macro index,pager Min "<save-message>=INBOX<enter>" "move mail to INBOX"
macro index,pager Cin "<copy-message>=INBOX<enter>" "copy mail to INBOX"

macro index,pager ise "<change-folder>=Sent<enter>" "go to Sent"
macro index,pager Mse "<save-message>=Sent<enter>" "move mail to Sent"
macro index,pager Cse "<copy-message>=Sent<enter>" "copy mail to Sent"

macro index,pager idr "<change-folder>=Drafts<enter>" "go to Drafts"
macro index,pager Mdr "<save-message>=Drafts<enter>" "move mail to Drafts"
macro index,pager Cdr "<copy-message>=Drafts<enter>" "copy mail to Drafts"

# Fetch mail shortcut
bind index G imap-fetch-mail

sidebar

# =================== SIDEBAR PATCH ==================== #
# l a y o u t
set sidebar_visible 	  = yes 
set sidebar_width   	  = 12 
set sidebar_short_path    = yes
set sidebar_next_new_wrap = yes
set mail_check_stats
set sidebar_format        = '%B%?F? [%F]?%* %?N?%N/? %?S?%S?'
set status_chars  	  = " *%A"
set status_format 	  = "───[ Folder: %f ]───[%r%m messages%?n? (%n new)?%?d? (%d to delete)?%?t? (%t tagged)? ]───%>─%?p?( %p postponed )?───"
color sidebar_new color221 color233

# b i n d i n g s
bind index,pager \Ck sidebar-prev
bind index,pager \Cj sidebar-next
bind index,pager \Co sidebar-open
bind index,pager \Cl sidebar-open
bind index,pager \Cp sidebar-prev-new
bind index,pager \Cn sidebar-next-new
macro index b "<enter-command>toggle sidebar_visible<enter>"    # b toggles sidebar visibility
macro pager b "<enter-command>toggle sidebar_visible<enter>"    # b toggles sidebar visibility

themes/custom

## ===================== COLORS ===================== #
# Default index colors
color index yellow default '.*'
color index_author red default '.*'
color index_number blue default
color index_subject cyan default '.*'
# New mail is boldened
color index brightyellow black "~N"
color index_author brightred black "~N"
color index_subject brightcyan black "~N"
# Other colors and aesthetic settings
mono bold bold
mono underline underline
mono indicator reverse
mono error bold
color normal default default
color indicator brightblack white
color sidebar_highlight red default
color sidebar_divider brightblack black
color sidebar_flagged red black
color sidebar_new green black
color normal brightyellow default
color error red default
color tilde black default
color message cyan default
color markers red white
color attachment white default
color search brightmagenta default
color status brightyellow black
color hdrdefault brightgreen default
color quoted green default
color quoted1 blue default
color quoted2 cyan default
color quoted3 yellow default
color quoted4 red default
color quoted5 brightred default
color signature brightgreen default
color bold black default
color underline black default
color normal default default
# Regex highlighting
color header blue default ".*"
color header brightmagenta default "^(From)"
color header brightcyan default "^(Subject)"
color header brightwhite default "^(CC|BCC)"
color body brightred default "[\-\.+_a-zA-Z0-9]+@[\-\.a-zA-Z0-9]+" # Email addresses
color body brightblue default "(https?|ftp)://[\-\.,/%~_:?&=\#a-zA-Z0-9]+" # URL
color body green default "\`[^\`]*\`" # Green text between ` and `
color body brightblue default "^# \.*" # Headings as bold blue
color body brightcyan default "^## \.*" # Subheadings as bold cyan
color body brightgreen default "^### \.*" # Subsubheadings as bold green
color body yellow default "^(\t| )*(-|\\*) \.*" # List items as yellow
color body brightcyan default "[;:][-o][)/(|]" # emoticons
color body brightcyan default "[;:][)(|]" # emoticons
color body brightcyan default "[ ][*][^*]*[*][ ]?" # more emoticon?
color body brightcyan default "[ ]?[*][^*]*[*][ ]" # more emoticon?
color body red default "(BAD signature)"
color body cyan default "(Good signature)"
color body brightblack default "^gpg: Good signature .*"
color body brightyellow default "^gpg: "
color body brightyellow red "^gpg: BAD signature from.*"
mono body bold "^gpg: Good signature"
mono body bold "^gpg: BAD signature from.*"
color body red default "([a-z][a-z0-9+-]*://(((([a-z0-9_.!~*'();:&=+$,-]|%[0-9a-f][0-9a-f])*@)?((([a-z0-9]([a-z0-9-]*[a-z0-9])?)\\.)*([a-z]([a-z0-9-]*[a-z0-9])?)\\.?|[0-9]+\\.[0-9]+\\.[0-9]+\\.[0-9]+)(:[0-9]+)?)|([a-z0-9_.!~*'()$,;:@&=+-]|%[0-9a-f][0-9a-f])+)(/([a-z0-9_.!~*'():@&=+$,-]|%[0-9a-f][0-9a-f])*(;([a-z0-9_.!~*'():@&=+$,-]|%[0-9a-f][0-9a-f])*)*(/([a-z0-9_.!~*'():@&=+$,-]|%[0-9a-f][0-9a-f])*(;([a-z0-9_.!~*'():@&=+$,-]|%[0-9a-f][0-9a-f])*)*)*)?(\\?([a-z0-9_.!~*'();/?:@&=+$,-]|%[0-9a-f][0-9a-f])*)?(#([a-z0-9_.!~*'();/?:@&=+$,-]|%[0-9a-f][0-9a-f])*)?|(www|ftp)\\.(([a-z0-9]([a-z0-9-]*[a-z0-9])?)\\.)*([a-z]([a-z0-9-]*[a-z0-9])?)\\.?(:[0-9]+)?(/([-a-z0-9_.!~*'():@&=+$,]|%[0-9a-f][0-9a-f])*(;([-a-z0-9_.!~*'():@&=+$,]|%[0-9a-f][0-9a-f])*)*(/([-a-z0-9_.!~*'():@&=+$,]|%[0-9a-f][0-9a-f])*(;([-a-z0-9_.!~*'():@&=+$,]|%[0-9a-f][0-9a-f])*)*)*)?(\\?([-a-z0-9_.!~*'();/?:@&=+$,]|%[0-9a-f][0-9a-f])*)?(#([-a-z0-9_.!~*'();/?:@&=+$,]|%[0-9a-f][0-9a-f])*)?)[^].,:;!)? \t\r\n<>\"]"

Which will result in something like that:

As a final note, I'd change the permissions of ~/msmtprc. ~/.mutt/muttrc, ~/.mutt/accounts/* and ~/.mutt/certificates to be 600.
That's all folks, enjoy!