What does GWIA log tell you?

Do you force secure SMTP?

Did you try to set a relay exception for that host (this is not an solution but helps us to analyze)?

Hi Diethmar.  Secure SSL is not required, nor is "force outbound authentication" ticked

Looking through the GWIA logs I can see onlky the following

12:19:23 FC79 DMN: MSG 8408616 Sending machine verification failed. (WH20221017)
12:19:23 FC79 DMN: MSG 8408616 SMTP session ended: [192.168.0.34] ()

If you check general internet addressing or only the user's internet addressing, is this one enabled?

I assume this is the reason 

This with a test telnet and an hostname in the EHLO command not being correct results in 

In the GWIA log

15:59:19 647F DMN: MSG 87 Accepted connection: [10.94.73.182] ()
15:59:24 647F DMN: MSG 87 Sending machine verification failed. (test.com)
15:59:24 647F DMN: MSG 87 SMTP session ended: [10.94.73.182] ()

So this is the same as you seem to report back to the application

Good point!

Thanks all - have received confirmation from the developer that  answer solved this