Hi, I'm an experienced Linux system administrator. My previous work before becoming freelancer was also take care of mail system of the company. Those are include STMP (exim4), IMAP (dovecot), and Web (roundcube) servers, all of them run on Debian Linux. All of those can handle starttls communication between servers and client. So, even though I might can't claim I have 'excellent knowledge' about email system, but I could claim that I understand them enough to handle them for many years.
So, I'm interested in your problem, could you please share more details of the problem in chat please.