Exchange 2019 - Periodic Email Delivery Issues in DAG

Question

What could be causing the periodic email delivery issues to internal mailboxes in a Database Availability Group (DAG) on Exchange 2019?

The situation involves two Exchange servers in the DAG where approximately once a month, there are problems with email delivery to mailboxes located on the neighboring DAG server. Each server has 50% of the databases running in active mode while the other half is in passive mode.

When a user on the EXCH1 server receives an email that arrives on the EXCH2 server, where the database is in passive mode, the email cannot be delivered to the user's mailbox. The Queue displays the following errors:

• {LED=451 4.4.395 Target host responded with error. -> 421 4.3.2 Service not available}
• {LED=451 4.4.395 Target host responded with error. -> 421 4.4.2 Connection dropped due to ConnectionReset}.

This issue persists for about an hour, after which the emails are delivered without problems and the queues dissipate. Notably, rebooting the server prevents the problem for a month before it reappears.

Exchange version: 15.02.1544.014.

TLS 1.0 and TLS 1.1 is disabled, TLS 1.2 enabled.