daknetworks.com

Watchguard, Mimecast and Office365

Couldn't get email from certain outside domains. Further investigation revealed that this is only happening from domains hosted at Office365. The error message in Mimecast is "Null result from socket."

This means that there is no response from the internal email server when Mimecast tries to deliver the message. That means it is being blocked by the WatchGuard.

WatchGuard logs show, something about the header size being 20656 and "header-line too large."

So WatchGuard is blocking anything where the header is too large.

You can see above the "Maximum email header size" is at 20,000 bytes.

We set it to: 21000.

Save > Push-Config

That did it!

NOTES:

http://www.watchguard.com/help/docs/wsm/xtm_11/en-us/content/en-us/proxies/smtp/proxy_smtp_gen_settings_c.html