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If you get an error message to the VPOP3 administrator with subject Router Problem and text
Local Distribute to <username> Failed Error: Database Error "ERROR: could not open file "base/<number>/<number>": Permission denied"
this error usually means that something is blocking access to that file. Usually this is a virus scanner, so you need to exclude the VPOP3\pgsql\data folder (and all subfolders) from any on-access virus scanning on the VPOP3 computer.
Often you will then need to restart the PC for any file lock which the virus scanner has already made to be released.
Once you have done this, you can recover the undeliverable messages by looking in the VPOP3\outqueue folder for BAD*.DAT files. Rename those to .EML files and you'll be able to open them up in most email clients to view them and/or forward them to the relevant people.