Não está incompleto. Você só sente falta dessa importante linha
See the section "Local recipient table format" below for a description of how the table should be populated.
nesse ponto, esta seção .
If you use local files in postmap(1) format, then local_recipient_maps expects the following table format:
In the left-hand side, specify a bare username, an "@domain.tld" wild-card, or specify a complete "[email protected]" address.
You have to specify something on the right-hand side of the table, but the value is ignored by local_recipient_maps.
Então o exemplo seria
nonexistaa dummyaccount
[email protected] dummyaccount