returns 0 row from plpgsql function

Dear List

I use plpgsql function as a postfix mailserver look-up table.  For
example to look up virtual email address.
If I don't host this email address, then the function returns null.
Because it's a 1 row of null value postfix logs a warning message.
Although it's harmless in that postfix takes no further action to
deliver mail, these warning messages fill up log table.

How can I return 0 row from plpgsql function?

Thanks

mr.wu