If VPOP3 cannot send messages using LAN Forwarding it will queue the message and retry periodically. Usually there is no need to do anything special if there was a problem with the onward mail server which has now been fixed, VPOP3 should soon retry the messages automatically (based on the value of the LAN Forwarding Retry Frequency setting in the LAN Forwarding configuration).
However, if you want to force a retry sooner, you can use the following SQL command.
If you want an extra bit of safety, see the running database updates within a transaction article.
At a command prompt in the VPOP3 directory run
(The password is 'vpop3pass')
UPDATE messages.outgoing SET nexttry=now();
N.B. VPOP3 does not need to be stopped while you do this