More inbound email failing with TT512 errors

[georgeh2011]

I've seen this a few times now with emails addressed to me from external sources.

This has got to be a TT error which needs fixed.

This seems to happen when my UPS (uninterruptible power supply) does it's weekly self test and sends two emails to me via one of my outlook.com accounts. So ALL of them are under my control.

At least three times I've received only one of them. Sometimes is is the first saying the self test has started. Sometimes the second, sent 1 minute later, saying it has passed.

The emails themselves look like this (this is the test started which I didn't receive, bounced by TalkTalk with the error shown above at the senders end):

And this is the corresponding "test passed" which was sent no more than a minute later and I received:

What is going on and what other emails (such as password resets) am I not receiving because of problems with TalkTalk?

[georgeh2011]

Hi Ady,

PM sent with lots more info.

[Ady-TalkTalk]

Hi georgeh2011, I've run a 7 day scan on your TT email address and there's nothing showing deferred or rejected in the last 7 days. If you let me have the sending address in a PM I'll check that too. 

Ady