Full-Quote of Guillaume's mail see below (mail was sent directly and
didn't go to the list).
My Opinion about this: Yes, you have to use dedicated submission-ports,
that's how it is defined to work. Misusing port 25 is a wideseen
configuration, but that's not how it was designed in the RFC's.
RFC6409 specifies port 587 as reserved, but allow port 25 to be used
also:
https://tools.ietf.org/html/rfc6409#section-3
"
Port 587 is reserved for email message submission as specified in
this document. Messages received on this port are defined to be
submissions. The protocol used is
