Email Stuck In Queue But Queue Already Empty

Dear everyone,

Here is the situation, we had some troubles with IIS logs and mailbox replication this morning which is causing lots of email to be stuck in queue. But after we clear it, some email seems to have gone missing. By checking using mail flow, all we got is "pending" and "email delivery is taking longer than expected. there may be system delay"

Can Exchange server drop the email if it is stuck in queue for too long?