I've been probing my XMail SMTP filter by sending spam to myself. XMail's logging is so-so. Here's XMail bouncing my spam.

""      "nXtXeXeXiX.com"        "XXX.XX.XXX.XX" "2004-10-05 13:00:17"
"kkkkk.nXtXeXeXiX.com"  "nXtXeXeXiX.com"        "zzzz@nXtXeXeXiX.com"
"aaaa@nXtXeXeXiX.com"   "SE6"   "RCPT=OK"       ""      "0"     ""

""      "nXtXeXeXiX.com"        "XXX.XX.XXX.XX" "2004-10-05 13:00:17"
"kkkkk.nXtXeXeXiX.com"  "nXtXeXeXiX.com"        "zzzz@nXtXeXeXiX.com"
"aaaa@nXtXeXeXiX.com"   "SE6"   "DATA=EFILTER"  ""      "0"     ""

The interesting parts are "RCPT=OK" and "DATA=EFILTER". This means the SMTP server was ok with the recipient but rejected the data.

Comparatively, Postfix is more informative:

Oct  5 13:00:16 kkkkk postfix/qmgr[175]: C17319D5: from=<>, size=3473,
nrcpt=1 (queue active)

Oct  5 13:00:17 kkkkk postfix/smtp[1156]: C17319D5:
to=<aaaa@nXtXeXeXiX.com>, relay=z.mx.nXtXeXeXiX.com[XXX.XXX.XX.XXX],
delay=46, status=bounced (host z.mx.nXtXeXeXiX.com[XXX.XXX.XX.XXX] said:
554 Transaction failed)

So XMail rejects mail by "554 Transaction failed".

What happened next was that Postfix tried to bounce the failure to the sender, which was me, meaning Postfix attempted to deliver the bounce to this particular XMail again. XMail 554'ed again, and Postfix junked the bounce, as specified by the protocol.