ttyp4:root@ibook 12:52:50/030527(/home/makoto)# grep hogesys /var/log/maillog
May 27 04:01:33 ibook sendmail[18239]: h4QIr2GY018239: collect: premature EOM: Connection reset by mon.hogesys.co.jp
May 27 04:01:34 ibook sendmail[18239]: h4QIr2GY018239: SYSERR(root): collect: I/O error on connection from mon.hogesys.co.jp, from=<mymy@hogesys.co.jp>
May 27 04:01:34 ibook sendmail[18239]: h4QIr2GY018239: from=<mymy@hogesys.co.jp>, size=1005, class=0, nrcpts=1, proto=SMTP, daemon=MTA, relay=mon.hogesys.co.jp [hh.221.yy.xxx]
May 27 12:36:11 ibook sendmail[19417]: h4R3RdGY019417: collect: premature EOM: Connection reset by mon.hogesys.co.jp
May 27 12:36:11 ibook sendmail[19417]: h4R3RdGY019417: SYSERR(root): collect: I/O error on connection from mon.hogesys.co.jp, from=<mymy@hogesys.co.jp>
May 27 12:36:11 ibook sendmail[19417]: h4R3RdGY019417: from=<mymy@hogesys.co.jp>, size=1005, class=0, nrcpts=1, proto=SMTP, daemon=MTA, relay=mon.hogesys.co.jp [hh.221.yy.xxx]

まさか、こういうのではないと思うけれど..

If you are using a Cisco firewall product that supports their fixup protocol,
try disabling it for smtp. As far as I know, it only supports SMTP and you may be using ESMTP.

ipnat の rdr 機能を使っていると、この問題があるという気がする。

ipf: IP Filter: v3.4.29 (344)
Kernel: IP Filter: v3.4.29 ~ Running: yes
Log Flags: 0 = none set
Default: pass all, Logging: available
Active list: 0

ではなくて ipnat の方か。

@(#)ipnat.c 1.9 6/5/96 (C) 1993 Darren Reed
@(#)Id: ipnat.c,v 2.16.2.21 2002/06/06 10:49:19 darrenr Exp