allow unencrypted reception (from the operator at least) and then send them a message about it
the chatmail homepage itself - the operator may disable IMAP for a day every week (or stop receiving new messages other than by the operator) and then most users would probably visit the webpage to see what’s happening
return IMAP/SMTP errors for some things, but most users won’t be able to find them (in the log or in the message info window)
Fediverse or any other contact form published on the web page