The actual reading of email with mu is about what's on disk. Using offlineimap is just a common configuration, and in its configuration you can specify a function, so if you wanted it to prompt you for a password whenever you get mail, you could do that. You could also just have Postfix deliver mail to your directory, then just have mu4e run the mu index command periodically to find new mail.
As for the writing of mail, it culminates in running some kind of sendmail program on your message. If you have sendmail or postfix setup locally to forward mail up to another server, you already don't need to have a password set up in Emacs. But you could also make that program do whatever you want, it just needs to be a binary that accepts certain arguments (or an emacs function that operates on the file).
As for the writing of mail, it culminates in running some kind of sendmail program on your message. If you have sendmail or postfix setup locally to forward mail up to another server, you already don't need to have a password set up in Emacs. But you could also make that program do whatever you want, it just needs to be a binary that accepts certain arguments (or an emacs function that operates on the file).