I had initially some authentication issues with my ISP Exchange SMTP
host ("530 5.7.1 Client was not authenticated")
If I'm not mistaken that is probably because the host-option doesn't
have a label for the credentials table. Your /etc/mail/secrets has to
look like this:
label1 user:password
and the host entry has to contain it like this:
action "relay" relay host smtp+tls://label1@exchange.example:587 auth
<secrets>
but when I switched to
their general purpose SMTP host, it worked fine.
They probably accept everything as long as the sending IP is from their
own range.
