On Tue, 19 Jun 2012, Graeme Fowler wrote:
> On Tue, 2012-06-19 at 15:24 +0100, Dennis Davis wrote:
> > *Extremely* wild stab in the dark here. Try reducing the MTU on
> > your mail relay(s) slightly. Say from 1500 to something like 1400
> > to 1450.
>
> Very good call. Prior to DATA, most transaction packets are far smaller
> than the MTU of most links in the chain. Post DATA, for messages where
> the total data size exceeds 1500 or so bytes, they are likely to be
> larger. If there's a device inline which is breaking PMTUd, or a poorly
> configured or maintained firewall (in the case of the NHS that was my
> experience) then sliding your MTU down might well help.
It is a good call, and I remember that incident.
I also recently read a post from TF about an investigation he did ... here
we are:
http://fanf.livejournal.com/74849.html
Spookily familiar (and I liked the second comment).
I did some ping -D with large packets to one of their outbound mailservers
and was successful up to a size of 1472 matching the 1500 interface MTU.
In the meantime, I've asked if they can get their provider to supply any
timeout settings on their smtp router, although they claim to be using
"standard" Exim settings.
If they really are, then I'll either look at reducing the delay time or
tweaking the interface MTU a bit.
Thanks for the suggestions,
Jethro.
. . . . . . . . . . . . . . . . . . . . . . . . .
Jethro R Binks, Network Manager,
Information Services Directorate, University Of Strathclyde, Glasgow, UK
The University of Strathclyde is a charitable body, registered in
Scotland, number SC015263.
