SMTP connection + lost while reading message data (header)

William Herrin bill at
Thu Sep 5 21:12:59 UTC 2013

On Thu, Sep 5, 2013 at 3:48 PM, Kwas Lee <leekwas at> wrote:
> Could someone please advice as some of sender are unable to send mails to my server when i check the logs  i found
> 2013-09-05 20:31:32 SMTP connection from ( [xx.xx.xx.xx] lost while reading message data (header)

Hi Lee,

Smells like path MTU. The connection died at what is probably the
first time in the SMTP session that the remote system tried to send
you a large, probably 1500 byte packet: the beginning (message header)
part of the data portion (the email message headers and body
themselves, as opposed to the sender and recipient declarations).

This is not the only candidate cause, but that's what my mind jumps
to. Try adjusting the TCP Maximum Segment Size on your server. Turn it
down to 1200 or so and see if the problem vanishes. If so, somebody's
losing ICMP packets.

Bill Herrin

William D. Herrin ................ herrin at  bill at
3005 Crane Dr. ...................... Web: <>
Falls Church, VA 22042-3004

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