Hi,
I have been getting this error on some emails in the SMTP delivery to mailbox queues. It seems to relate to this message in event viewer.
Agent 'Mailbox Rules Agent' caused an unhandled exception 'StoreDriverAgentTransientException: Store Driver Agent was unable to send email.' while handling event 'OnDeliveredMessage'
The email does seem to get delivered but it can sit in the queue for a long time. It doesn't affect everything either.
I do have a rule to drop read receipts as is was causing problems when importing user pst's.
The server is Exchange 2013 CU2 coexisting with 2010. I'm in the last stages before removing 2010.