libetpan is required for nmail and other email clients (I think claws-mail uses it too). After discussion with the nmail developer, it was found that libetpan on Solus is built without libsasl2 support, which prevents sending emails. This seems like a core feature and could be fixed easily by just rebuilding libetpan from sources.

For reference, the description and tests confirming this issue are in the few messages starting here: https://github.com/d99kris/nmail/issues/28#issuecomment-583748673