Determining deliverability

[Jason Fesler]

> Or do you want your MTA to reject the message during the SMTP session and
> not bounce it later?
>
> If you want the later situation, you might be stuck.

Just occasionaly dump a list of users, and feed it into the MTA's favorite
format.  Or do it after "newuser" gets ran.  Or whatever frequency/event
makes sense.  Nothing says that _cyrus_ has to be queried on every "RCPT
TO:"..