aegee-milter was developed for a mail server that served a lot of aliases redirecting to different email providers, and mailing lists with mailboxes on different email providers. The challenge was to keep the IP reputation good. This means not to forward received spam, recognized as such by the anti-spam software and not to send bounces. Bounces were sent from the mailing list manager e.g. when a sender uses an address for posting which cannot post to the mailing list.
The further, some mailboxes received more spam than the others, so their spam filter had to be set more aggressively. Thus one mail sent simultaneosly to an address with more aggressive spam filter and with less aggressive spam filter had to be rejected for the first recient and accepted for the second.
When the author of aegee-milter took over a mail server and installed anti-spam software, he modified the mailing lists to bounce emails evaluated as spam.
The mail server