Fortimail - 421 4.7.0 Too many concurrent connections

Hi All, 

Brand new here :)

I need a hand to an issue I am having with my Fortimail appliance, where large volume emails (legitimate!) are being refused by Fortimail.

Error: 

Deferred Queue              (host %FORTIMAIL%[%FORTIMAIL IP%] refused to talk to me: 421 4.7.0 Too many concurrent connections.)

This is doing my head in, as I cannot find the option anywhere to increase throttling!

Fortimail: 400E

My Session and SMTP limits are put to the maximum, but the issue still persist. I keep reading the cookbook, but I cannot find where to increase throttling overall, or for a particular SMTP address (That would be FireEye in our case).

Any ideas or pointers?

Many thanks, 

Fred