Error 550 5.1.10 when sending from local smtp (hybrid) and no error when sending from OWA

Question

We have a hybrid setup where all mailboxes are located in EO, and exchange server runs in Hybrid mode with local SMTP.

We have no problems sending an e-mail to an address sample@Company portal .com (recipient is outside of our tenant), but when we send a mail from local SMTP on Exchange Server we get

> Remote Server returned '550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup'

"Undeliverable" mail generating server is Exchange Server, so it seems the mail doesn't even leave on-prem to EO, however I checked Hub Logging logs and found no errors.
What could be a reason of that mail failure?

Answer 1

is Sample@Company portal .com an account in your org?

If so, then its one of two issues:

1. There is no remote mailbox enabled for that account on the on-prem Exchange Server

or

1. The remote routing address on the existing remote mailbox on-prem is set incorrectly.

Answer 2

@Mr. Peterson

In a hybrid organization, if you send email from Exchange on-premises, email will be sent from Exchange on-premises to the Internet directly.

So, I would suggest you create a new send connector for Exchange on-premises: Create a Send connector in Exchange Server to send mail to the internet

Then enter this external email address manually (Don't use auto-complete or reply old emails)

If you still cannot send emails successfully, I would suggest you try to use Message Tracking Log to check which step has issue:

Get-TransportService | Get-MessageTrackingLog -Sender user@domain.com -Recipients externaluser@domain.com -MessageSubject "subject" -Start 09/22/2022 -End 09/24/2022  

---

If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.

---