i submitted a patch to /n/sources/plan9/mail/lib/validateaddress
which geoff put in. it turns out this patch is quite a bit more
important than i thought. without it, upas is an open spam relay.

it works like this. spammer sends mail with a forged From: line
and a To: line that mail -x will flag as an "Invalid address". the old
script would not flag this for rejection and upas would send a
failure notice to the sender, thus spamming the guy in the forged
From: line.

- erik