If VPOP3 v3 or later won't start, and you find that the VPOP3DB (or PostgreSQL-VPOP3) service does not exist in the Windows Services list, it may be that you have just moved VPOP3 and made a mistake following the move procedure or something has removed the service for some reason.
The normal reason is that you have copied the VPOP3 folders from another PC, but forgot to remove the VPOP3\pgsql folder before reinstalling VPOP3. In this case, the best option may be to rename the VPOP3\pgsql folder to pgsql_old, then reinstall VPOP3 again and then restore the database backup from the old PC. (Once you are happy that all is working OK, you can delete the pgsql_old folder.)
If this is not possible for some reason, you could try adding the VPOP3DB service manually
For VPOP3 v5 and later you need to:
pg_ctl register -N VPOP3DB -U vpop3postgres -P Nc6ACboDt2jVL6 -D “c:\vpop3\pgsql\data” -w -t 300