I'm having problems with receiving some mails on a QMail box. To be more specific, only mails with rather larger attachments (+500K). This appears to be a firewall issue, because when the firewall (IPF) is off, the message is received.
I kept track of the blocked packages and saw the following packet being blocked during such SMTP transfer:
09/05/2008 12:54:23.973302 xl1 @0:28 b 220.127.116.11,58211 -> 18.104.22.168,25 PR tcp len 20 1500 -AP IN OOW
A packet gets Out Of Window and thus is blocked.
I read that this is a IPF issue, which might be fixed in a later versions (running IPF 4.1.8 (416) now). freebsd-update didn't offer IPF updates and I'm not feeling like I'm going to upgrade that server to a newer FreeBSD (running 6.0 now).
What do you suggest, is there a fix without doing updates?