O365 Connector - Validation OK, Delivery from On-Prem OK, but Cannot Send to On-Prem

Question

O365 Connector - Validation OK, Delivery from On-Prem OK, but Cannot Send to On-Prem

Edwards Lie 1

I have tried setting up both Inbound and Outbound connectors between O365 and my On-Prem server. Setting connection from O365 to On-Prem was validated successfully, and setting connection from On-Prem to O365 was also successful. Also, we have performed sending test email from On-Prem email account out to a personal email on Gmail and Outlook and was sent successfully. However, we cannot reply to those email at all. Everytime we reply those email, we've received NDR error 554. 5.4.14. All the setup are below:

How do I fix this? The On-Prem is our CPanel-based Webmail.

Thanks.

Andy David - MVP 157.8K Reputation points

2022-02-20T16:04:54.967+00:00

Do the headers of the message indicate its actually getting routed back from to on-prem and 365?
Does this happen with all on-prem mailboxes?

Is there an Edge Role server in the mix or anything other server these may be routing through other then on-prem Exchange?
Edwards Lie 1 Reputation point

2022-02-20T16:34:19.537+00:00

To clarify, we're not using Exchange on the On-Prem server. We're using what is supplied by our web hoster, which is a Sendmail system, managed through cPanel of our hosting account.

Do the headers of the message indicate its actually getting routed back from to on-prem and 365?

I haven't dig deep into the header yet, but hope this excerpt give you some insight:

It seems that the O365 mail server never actually attempted to call to the IMAP server of ours to route the email back to the on-prem server. Which is odd, since the validation was successful.
Come to think of it, isn't a successful validation on O365 -> On-Prem will result in an test email in the validated email's inbox? I never seen one in our on-prem inbox even though the validation was successful.

Does this happen with all on-prem mailboxes?

Yes.

Is there an Edge Role server in the mix or anything other server these may be routing through other then on-prem Exchange?

No, it's just straight up (internet) <-> O365/EAS <-> on-prem Sendmail server. Nothing in between O365 and on-prem.

1 answer

Your answer

Andy David - MVP 157.8K Reputation points

2022-02-20T16:04:54.967+00:00

Do the headers of the message indicate its actually getting routed back from to on-prem and 365?
Does this happen with all on-prem mailboxes?

Is there an Edge Role server in the mix or anything other server these may be routing through other then on-prem Exchange?
Edwards Lie 1 Reputation point

2022-02-20T16:34:19.537+00:00

To clarify, we're not using Exchange on the On-Prem server. We're using what is supplied by our web hoster, which is a Sendmail system, managed through cPanel of our hosting account.

Do the headers of the message indicate its actually getting routed back from to on-prem and 365?

I haven't dig deep into the header yet, but hope this excerpt give you some insight:

It seems that the O365 mail server never actually attempted to call to the IMAP server of ours to route the email back to the on-prem server. Which is odd, since the validation was successful.
Come to think of it, isn't a successful validation on O365 -> On-Prem will result in an test email in the validated email's inbox? I never seen one in our on-prem inbox even though the validation was successful.

Does this happen with all on-prem mailboxes?

Yes.

Is there an Edge Role server in the mix or anything other server these may be routing through other then on-prem Exchange?

No, it's just straight up (internet) <-> O365/EAS <-> on-prem Sendmail server. Nothing in between O365 and on-prem.

Answer 1

Andy David - MVP 157.8K

Ok, so we know its never actually making it to the on-prem server then...
Are the domains set right in that connector? Can you set to the specific domain instead of a wildcard?

Edwards Lie 1 Reputation point

2022-02-20T16:49:29.987+00:00

I've tried removing the wildcard, yet still to no avail.
Edwards Lie 1 Reputation point

2022-02-20T16:54:52.313+00:00

Can I send you the screenshot of my outbound (O365 to on-prem) connector setup privately?

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O365 Connector - Validation OK, Delivery from On-Prem OK, but Cannot Send to On-Prem

1 answer

Your answer