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Date: Wed, 26 Mar 2014 09:23:42 -0700
From: Gregory Shapiro <gshapiro@freebsd.org>
To: Willy Offermans <Willy@Offermans.Rompen.nl>, freebsd-current@freebsd.org
Subject: Re: sendmail Broken Pipe Error
Message-ID: <20140326162342.GC72635@C02KN7GSDTY3.corp.proofpoint.com>
References: <20140325103934.GH8104@vpn.offrom.nl>
 <20140325164316.GC32089@funkthat.com>
 <20140326111748.GJ27307@vpn.offrom.nl>
 <20140326162034.GI60889@funkthat.com>
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> > 11:57:56.539788 IP MyServer.com.41115 > Smarthost.com.smtp: Flags [S], seq 1001452351, win 65535, options [mss 1448,nop,wscale 6,sackOK,TS val 407239960 ecr 0], length 0
> > 11:57:56.555262 IP Smarthost.com.smtp > MyServer.com.41115: Flags [S.], seq 1277075046, ack 1001452352, win 8192, options [mss 1452], length 0
> 
> It should look something like:
> 09:18:34.723280 IP jmgmac.funkthat.com.64724 > h2.funkthat.com.ssh: Flags [.], ack 177, win 33280, options [nop,nop,TS val 1854905469 ecr 3482476972], length 0
>         0x0000:  4510 0034 d7ac 4000 4006 e1af c0a8 0003  E..4..@.@.......
>         0x0010:  c0a8 0004 fcd4 0016 7e48 238e d872 43dc  ........~H#..rC.
>         0x0020:  8010 8200 7c08 0000 0101 080a 6e8f 9c7d  ....|.......n..}
>         0x0030:  cf92 61ac                                ..a.
> 
> Notice the hex output...  I didn't see any of that in your output...
> The last packet I was talking about is the last one that had length
> 1448 that your server sent...

Willy mentioned that this only happened on mail with attachments.  Could it be an MTU issue?  Specifically, see:

http://www.sendmail.com/sm/open_source/tips/path_mtu/