On my FreeBSD laptop I’m used to read and answer the mails with the MUA mutt. I can do the same, on the L5 reading mails with IMAPS from my server. To answer it would be good to have some MTA on the L5, i.e. sendmail or postfix. This would allow to answer mails even offline, the mail gets queued and sent when Internet is available again.
Has someone done such MTA configuration on the L5?
While I typically send emails directly from mutt to my mail server (now that mutt supports that) instead of using a local MTA, it would be no different to configure postfix on the L5 than it would on any other Linux computer running PureOS or Debian.
I wrote a guide about 10 years ago for Linux Journal for how to set that up with postfix that should still work today: https://www.linuxjournal.com/magazine/hack-and-make-local-mutt-mail-server
If you attempt to send the email direct from the Librem 5 to the recipient, you will typically have no end of problems with your email being rejected and/or blocked, particularly if your Librem 5 comes back onto the internet via its mobile connection, rather than its WiFi connection.
So I would suppose that you would have to configure to send to a carefully chosen mail relay server once the Librem 5 comes back onto the internet and let that mail relay server deliver to the actual recipient.
This is theoretical though i.e. I have not tried this (yet). So it may work better or worse than I am predicting. 
I have an ISP which apart of web space (http://www.unixarea.de) gives me SSH, IMAPS and SMPT access. Normally, on my FreeBSD laptop, I fetch down my mails with fetchmail, scan them via procmailand SpamAssassin before they get to /var/mail/guru. Outbound mutt sends the mail via sendmail to the local queue where a local MTA sendmail delivers the mail to my ISP for further MX chaining. This works all nicely for 20 years, even when I’m in Havana, Cuba, with my laptop.
As I now have with the L5 a real Debian Linux “server” in pocket format, I will try to setup this too in the L5, together with some rsync procedure to keep the various mbox files in sync.
That is the key point. You are not sending the email direct from your computer to the recipient.
For your usecase, a lighter MTA such as nullmailer should do perfectly well.
Basically, it only sends email to another smtp relay and handle a local queue.
It exposes sendmail interface locally, tbc but I would imagine mutt can use that.
Many folks have had success using offlineimap for that purpose, however if you already have another system you like I’d probably just stick with that and reuse all of that work.