If you are finding that email users using Mail.app cannot send mail, and that you are getting log lines for that user in mail.log which look like this (from a Postfix system):
postfix/smtpd: connect from host.name [x.x.x.x]
postfix/smtpd: SSL_accept error from host.name [x.x.x.x]: -1
postfix/smtpd: lost connection after STARTTLS from host.name [x.x.x.x]
postfix/smtpd: disconnect from host.name [x.x.x.x]
And this machine is also running a POP or IMAP server, then it is probably because the SMTP server and IMAP server have different certificates in use, which Apple Mail has issues with if they share a hostname. This seems to be the default situation often in the Debian packages. Aim the two at the same certificate and key file, and all shall likely be well.
The vastest things are those we may not learn.
We are not taught to die, nor to be born,
Nor how to burn
How pitiful is our enforced return
To those small things we are the masters of.
This is the wineglass in which red wine was served to Tycho Brahe just before he died, four hundred and more years ago. The astronomer's body was exhumed just a few years ago to try to find out how he died, but still nobody is quite sure. Rumours of extramarital intrigue.
This is the wineglass which ended up in a rubbish dump, dug up a few hundred years later, intact and shining, in a back garden where there were no worms. Things grew better in the garden after it was removed; it was washed, and put away.
This is the wineglass which makes its way through a succession of new couples' drink cabinets; it always shines, and is always someone's favourite. Afterwards, it moves on; it is thrown away, or given to a charity shop; and it repeats again.
This is the wineglass which turns everything poured into it into some kind of poison, slow-acting or fast-acting as it wishes. It leaves behind it a trail of grieving, arrests and the lonely. It decides what happens; and there is no reason for what it chooses.
This is the wineglass that will only accept red wine.