When I was a Solaris admin..
.. I *hated* Exchange 5.5..
All of our network was Solaris/Sendmail except for one office that had mysteriously migrated to Exchange 5.5 overnight. Mail looked like it was still flowing so everyone relaxed.
Until the director of that office complained that he wasn't getting some emails. And it seemed to be a random assortment that he was missing. This was pre major search engines so I spent a lot of time trying to debug the issue. I finally found a found on an obscure forum where someone was complaining about failed SMTP pipelining.
Sendmail, on contacting the destination server, would parse the server capability header response and tailor the session accordingly. And Exchange 5.5, coming from that bastion of conformity to standards that late 1990's Microsoft was, proclaimed that it could use pipelinging (ie - one SMTP session could accept multiple emails). Which was fine except that it couldn't - or at least, not from sendmail.
Fortunately, with sendmail being an insanely complex beast, it could be configured on a per-destination server to over-ride the declared capabilities and, once we told it to *not* use pipelining with that server, all the mails started being received. It took more traffic though and, since we were all on frame relay, more cost.