Here is a configuration to make Neomutt more or less work for reading Chatmails on a mobile formfactor. This is useful for debugging (you can see the subjects and control e-mails easily, and archive messages).
Extract your private keys from Deltachat (there is an option for this in the GUI, except in the Android client). Add them to your keyring; installing Seahorse gives you a keyring GUI which works moderately well on mobile, and is often faster than typing. Export your e-mail address and password, too.
Instal Neomutt and set up your account configuration. You may want to use mutt-wizard. Chatmail accounts autoconfigure pretty neatly (though Neomutt does not have the simplest config).
Run “mbsync -a” to sync with the server IMAP folders (“mbsync -h” for more options). Chatmail servers auto-delete messages older than 30 days by default (deadline varies, see your server’s terms page). If you wish to retain old mails locally after the server deletes them, run someting like mbsync -Lan Sync; optionally, automate as an Anacron job.
If you are on a computer with a full-sized screen, fairly standard Neomutt config should do, but if you are on mobile Linux, with a pocket-size touchscreen, the default display options will be pretty unusable.
In your muttrc file, add this configuration:
#Fit mobile formfactor
set sidebar_visible=no #can be toggled with "B"
set sidebar_width = 20
set date_format="%y-%m-%d/%H:%M"
#%F is ISO date
set index_format="%2C%Z %D%s %?X?A&?%-15.15F %> (%-4.4c)"
Open a mobile-compatible terminal program, like kgx (KGX is currently the program that Gnome calls “Console” or “gnome-console”, but this may change). Run “neomutt”, and then toggle CTRL on and type “-” repeatedly until the text is a suitable size for you. Now you can play with the muttrc config until you like the look of the display.
Note that this will receive e-mails, but not send them. It should be possible to configure sending; see Chatmail with Neomutt.