For those using Sophos SPX Email Appliance:
One of my customers kept getting emails stuck in the appliance queue to yahoo, btinternet and aol email addresses with status of Delivery Deferred. The reason given in the log was either the domain was not found or connection lost.
Using the diagnostics of the appliance the DNS and ping tests were fine. Checked email blacklists and reserve DNS were ok.
The appliance sits behind a Draytek load balancer on PPPoE mode to a Linksys WAG320N on ADSL2+ connection.
After adjusting the Draytek MTU size on the WAN port to 1400 from the default of 1492, the appliance queue cleared instantly.
No idea why this suddenly happened as the setup has been operational for months so thought I'd post it on here just in case anyone else experienced the issue. My customer is based in the UK.
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