Every time VPOP3 adds a message to the Outgoing mail queue, it runs a script called outmessage.lua.
Scripts are stored as files in the VPOP3 directory in VPOP3 v6.6 and earlier, or managed through the Settings → Scripts page in VPOP3 v6.7 and later.
See the Lua Scripting page for general details about VPOP3's scripting, and information on how we can help.
For every message which is to be sent VPOP3 calls a function in that script called NewMessage.
The function signature is: NewMessage(AuthenticatedSender, ReturnPath, CreationDate, Subject, SenderIPAddress, FileLength, Priority, Headers, Message, HoldSeconds, DeleteAfterSeconds)
The function returns a table of new settings. This can contain the following fields. If the field doesn't exist, then the value is not changed.