I cannot receive external emails after migrating from Exchange Server 2013 to 2019.

Hello everyone,

I just followed the step-by-step guide from the following link: Migrating from Exchange 2013 to Exchange 2019 - A Step-by-Step Guide to migrate my Exchange 2013 server to the 2019 version. I completed all the steps mentioned, including creating the connectors, databases, etc. Currently, I have both servers coexisting.

I used a test mailbox, which I migrated to one of the databases on the new 2019 server. The problem I'm encountering is that when sending an email from this mailbox to outside the organization, the emails are received. However, when I reply to that email from outside, the replies are not received in the Exchange 2019 mailbox.

I used the Microsoft Remote Connectivity Analyzer tool, and it didn't show any errors. When I check the queue on the 2013 server, I can see that all the emails sent from outside are in it. The following error is indicated:

Identity: UHPEX2013\Unreachable\309869005505088
Subject: RE: Test
Internet Message ID: <email address removed for privacy reasons>
From Address: email address removed for privacy reasons
Status: Ready
Size (KB): 413
Message Source Name: SMTP:Default UHPEX2013
Source IP: 200.123.132.100
SCL: 0
Date Received: 23/05/2024 02:26:42 PM
Expiration Time: 25/05/2024 02:26:42 PM
Last Error: There is currently no route to the mailbox database.
Queue ID: UHPEX2013\Unreachable
Recipients: email address removed for privacy reasons;2;3;There is currently no route to the mailbox database.;2;ExternosUHP2EX2019;0.

Could you please help me resolve the issue without causing impact or disruption to my productive 2013 server?