I recently updated my Exchange 2016 server running on 2016 Standard Server OS and since the upgrade I have been seeing the SmtpDeliveryToMailbox queue status in Retry. I checked the event logs and did find a Certificate error which I fixed using the Error logs suggested command then rebooted the server and mail deliveries worked until sometime last night. This morning the queue is again stuck in Retry status. I have verified my server has the Oct and Nov Security updates released by MS as well. Outgoing emails are flowing without issue. This affects external emails coming in to us and internal emails between mailboxes.
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Did you try recreating certificates by running new-exchangecertificate?
Restart transport service and check.
The largest memory consumer on the server was the Exchange Store Worker Process but it is always consuming a fair chunk of memory.
that is by design
Exchange and SQL will use as much memory as it has available to it.
can you correlate anything in the application log at about the time the queue issue occurs?
how is the disk space on C? once the volume reaches about 20% free the transport service will stop and will cause issues with mail flow though might not be an issue if it is fixed with a reboot
I had to reboot the server every morning since the upgrade last monday evening, 8 Nov 2021, to get the email delivery to the mailbox queue working again. However since Thursdays reboot I haven't had to reboot and the delivery queue is still working as of now.
Hopefully it will continue working so this issue may have cleared itself up on its own. There was nothing obviously related in the event logs once I remedied the certificate issue last Tuesday morning. There were a number of KB updates that came down last week so one of them may have also fixed this issue.