'451 unable to verify user' on default-non-local-route

[Rick te Kronnie]

I'm currently troubleshooting an issue we have with our Messaging Gateway, running as a virtual appliance on version 10.7.4-13

We process over 3000 e-mails a day with no issue. But we are having problems with sending e-mail to one of our customers. Our users get a 'delivery delayed' message in their Outlook, but not always.

I've checked all e-mail sent to that domain in the last 30 days, made an export and sorted them by amount of delivery attempts. There's 46 e-mails sent from multiple users to that domain, it's mostly 1 recipient, we haven't tried other recipients yet.

1 e-mail never arrived
4 e-mails took 8 attempts before delivery
2 e-mails took 7 attempts
1 e-mail took 5 attempts
3 e-mails took 4 attempts
2 e-mails took 3 attempts
7 e-mails took 2 attempts
17 e-mails arrived on first try

The only thing I see in the Audit Logs is this multiple times for each attempt

Transient Delivery Attempts	Failure Time	Details	Recipient
Attempted Delivery to: default-non-local-route	Thursday 10 Jun 2021 03:53:22 PM CEST	451 unable to verify user	[e-mail]
Attempted Delivery to: default-non-local-route	Thursday 10 Jun 2021 04:10:22 PM CEST	451 unable to verify user	[e-mail]
Attempted Delivery to: default-non-local-route	Thursday 10 Jun 2021 04:42:22 PM CEST	451 unable to verify user	[e-mail]
Attempted Delivery to: default-non-local-route	Thursday 10 Jun 2021 05:44:22 PM CEST	451 unable to verify user	[e-mail]
Attempted Delivery to: default-non-local-route	Thursday 10 Jun 2021 07:46:22 PM CEST	451 unable to verify user	[e-mail]
Attempted Delivery to: default-non-local-route	Thursday 10 Jun 2021 11:48:23 PM CEST	451 unable to verify user	[e-mail]
Attempted Delivery to: default-non-local-route	Friday 11 Jun 2021 06:50:23 AM CEST	451 unable to verify user	[e-mail]

It also seems completly random, the issue persists regardless of time of day or if there are attachments or e-mail size. I've send an e-mail to them myself (log above) to get into contact with their IT department and the e-mail only had 1 .png file from my signature and it's not been delivered yet.

[alexander-smg]

Your issue is that the next hop server no like your smg. Make sure you check your gateway up if your blacklisted. Make sure any email to that domain is consistently failing or being delivered. And make sure you tell us the exact ip you are sending to so “I” can verify what u r saying from MY smg.

If I can send, then we are good. If I can’t send to your customer, then bigger issues linger.

Remember this: even though u think that your 3000 emails are delivered, it don’t mean that YOUR smg public ip has a good reputation!!!!!!!!

[alexander-smg]

Let’s try again. What is the malquery logs show. And what is the mx records your dns shows for this recipient domain?

[Rick te Kronnie]

Hi Alexander, thank you first of all for responding and sorry for the late response.

The customer told me the problem wasn't on their end but I contacted the IT company that hosts their environment and there was an IP-address set wrong in their spamfilter. The issue has been resolved. I just got a response from them late last night.

[alexander-smg]

See