Hello 
This is explained in the Exim documentation:
It means that there was a timeout while Exim was reading the contents of a message on an incoming SMTP connection. That is, it had successfully accepted a MAIL command, one or more RCPT commands, and a DATA command, and was in the process of reading the data itself. The length of timeout is controlled by the smtp_receive_timeout option.
If you get this error regularly, the cause may be incorrect handling of large packets by a router or firewall. The maximum size of a packet is restricted on some links; routers should split packets that are larger. There is a feature called “path MTU discovery” that enables a sender to discover the maximum packet size over an entire path (multiple Internet links). This can be broken by misconfigured firewalls and routers. There is a good explanation at http://www.netheaven.com/pmtu.html. Reducing the MTU on your local network can sometimes work round this problem. See Q0017 (3) for further discussion.
Thank you.
This is explained in the Exim documentation:
It means that there was a timeout while Exim was reading the contents of a message on an incoming SMTP connection. That is, it had successfully accepted a MAIL command, one or more RCPT commands, and a DATA command, and was in the process of reading the data itself. The length of timeout is controlled by the smtp_receive_timeout option.
If you get this error regularly, the cause may be incorrect handling of large packets by a router or firewall. The maximum size of a packet is restricted on some links; routers should split packets that are larger. There is a feature called “path MTU discovery” that enables a sender to discover the maximum packet size over an entire path (multiple Internet links). This can be broken by misconfigured firewalls and routers. There is a good explanation at http://www.netheaven.com/pmtu.html. Reducing the MTU on your local network can sometimes work round this problem. See Q0017 (3) for further discussion.
Thank you.
For anyone landing on this page, MTU is important but so is MSS CLAMP. So if you are on a pppoe connection, the eth should have a MTU of 1500, pppoe 1492 and the MSS clamp should be 1412.