Some emails for one employee are getting stuck in the sent items

Some emails for one employee are getting stuck in the sent items

I can't figure out any reason for this, some of my emails are getting stuck in the sent items and are marked as "In Queue".  On of the emails is from yesterday.

Other mails go through ok, to various addresses.  One of the emails stuck is going to my domain to a distro group...

Is there any method to attempt to flush the queue or retry again sooner than whatever the schedule is?