[Exim] Mail not being sent

Hi,
We have a situation where one of our customers is using Exim for sending
mail and on occasion it seems to not be sending email out. The first time I
noticed this there was about 3 Frozen emails at the top of the queue and 30
non-frozen. I deleted the 3 frozen and the rest started to be sent.
Now this has happened again, this time the customer says that there were no
frozen emails but when he forced the top email back to the sender the rest
of the queue started to go through ok.
My question is, is there a way to delete mail automatically if it sits in
the mail queue, say 4 or 8 hours and send it back to the user asking them to
resend their message, either by a script or within Exim. Also why would one
or two emails stop the rest of the emails going through.
Version output information: Exim version 3.22 #1 built 24-Jan-2001 00:15:58
Thanks in advance.
Wayne