tl;dr do not try to interoperate with non-Delta-Chat-clients
Delta Chat uses imap/smtp under the hood. this means, that you can use chatmail relays (highly recommended). if you really need to, you can also use other imap-/smtp-servers, eg. your own, as long is you use the account exclusively for chatting.
apart from that, however, using Delta Chat to communicate with traditional email accounts is strongly discouraged since several years. even if you manage to configure thunderbird to deal with encrypted emails, you will get a lot of noise that is not hidden by thunderbird. and you would push that noise and effort to other users.
some years before - we tried to play the card “you can write to any email address”. while that obviously still sounds interesting to some, it never took off. there were too many cumbersome things. also as both, classic email provider/clients were not really moving nor had much interest.
since we focus on encryption and simpleness, we got a by far larger userbase.
finally: if you even want to process unencrypted mail with Delta Chat, please just use an app that focuses on that, as Fairmail, Thunderbird etc..