What does the error SMTP data timeout (message abandoned) on connection from... mean?

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 for further discussion.