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.
