I have multiple contenders ;)

Contender one:

A program used to sort emails.

We was in the process of moving from lotus notes to exchange and needed a way route emails to the right server internally.

Solution, a qmail to receive all emails, a script running by cron every minute to read the emails, check the recipient name to a list and resending to the right server. The script was written in php :P since that was the only way we at the time had to read an email into an object, it was run just like any other shell script :D

Contender two:

A multi threaded mail sender that fetch email addresses and content from a database and posted them through qmail using background execution and pipes to get the result back and then update the database, written in bash script.

Contender three:

A c program used in a similar way as in one but this time using dial up and uucp to fetch email and then drop these either into lotus notes or into a bbs for our customers to give them an email address. This was around 1993, so not to many isp’s offered email and not to many had internet anyway, dial up bbs was much more common.