[Mimedefang] check_against_smtp caching?

[Michael Faurot]

> That's not hard to do with a tied db in mimedefang-filter.

This is a cool idea and wasn't very difficult to put together using a
tied db file.  I use the recipient address converted to all lowercase and
striped of angle brackets to act as the key.  For the value, I store a
positive timestamp if the recipient actually exists on the remote server
and a negative timestamp if it doesn't.  I'm then using an external Perl
program run out of cron to periodically expire things.