Watchguard SMTP Proxy Rejecting e-mails with error 500 Line Too Long

Some senders get a 500 error message after being dropped by Watchguard SMTP Incoming Proxy rule with the following error: SMTP error from remote mail server after end of data: host xxxx [1.2.3.4]: 500 Line too long

A few customers have reported us some issues with retreiving E-Mails from outsiders. The sender was getting back an undelivery message stating the message has not been sent due to a hard error 500 Line Too Long.

It turns out after an analysis that the senders were using Microsoft Outlook 2013 E-Mail client. It also turns out that the HTML formatted body is compiled by the client to be entirely on the same line. Thus, a 4000 bytes body message can easily have a line length of more than 4000 characters, which exceeds the 1000 bytes default SMTP Proxy limit.

We found out that even setting the value to 8000 or 12000 was useless, as the e-mail messages were getting dropped as well (before OLK2013 we have used 1536 line length by default because some CAs used to send messages with 1100 characters long).

Sadly at the moment the only way to solve this issue (quick and dirty) was to disable the "Maximum email length" checkbox in the SMTP Proxy policy setting. I will be debugging this problem a bit more to determine whether there is a neater and more secure way to handle this exception.