SMTP data timeout after cPanel upgrade

Since the update to to 11.52 Build 18, I've been unable to receive emails from certain servers. The message times out with an error of :

2015-10-26 09:22:26 SMTP data timeout (message abandoned) on connection from ********.*******.com [***.***.***.***]:56423 F=<********@***********.com>

I get them from multiple different senders, and the email never seems to bounce, instead continually retrying to send the same mesage.

 

Just an update, I was able to solve this by lowering my MTU. I'm not sure exactly what the problem was in the first place, but after changing it, email came flooding into my server.