Soft Bounce - Received too many messages from new or untrusted IP

[Christiane_Rode]

I've been monitoring our email bounces, and for one of our most recent email sends, we had a large number of bounces with the bounce reason: 

554 5.4.7 [internal] (last transfail: 451 4.7.1 Received too many messages from a new or untrusted IP

We are on a dedicated IP and have been for over a year and a half, so our IP isn't new, and I haven't seen this code before on our other reports. Has anyone else seen this creep up on their end?

There were a few companies that appeared more than others, so my thought is that maybe they changed some of their internal settings, but I can't seem to understand why we saw a 300% increase in bounces. 

Any insights are greatly appreciated.

[SanfordWhiteman · Accepted solution]

This means affected customers hardened the settings on their mail security service (or the service changed its global settings).

It's not so much that you are new, but you are new to them with regard to this security feature.