ActiveSMS will queue message indefinitely if the SMTP server is not available.
However, if the SMTP server is available but is rejecting emails then messages will be removed from the queue.
If the messages are no longer in the Incoming Queue (as indicated on the ActiveSMS Console page) then it is not possible to re-process the message.
On the ActiveSMS Console, on the Status Page check the 'Queued Incoming Messages', if this is 0 than the message are no longer queued and cannot be re-processed.