If you receive the messages after adding in the domain on the approved
senders list (ASL), though not recommended, that means messages or at
least the full domain are not being blocked entirely and most likely
in Postini's quarantine (if used) or Gmail's spam folder.
When you tested this before adding the domain to the ASL, did you
receive any bounce return error messages? You may try changing
Postini's blatant spam blocking (BSB) to return an error instead of
blackholing the message. Try this without the domain in the ASL.