Looking for an Email Program

Hello,
i look for an email client that must:

1) have automatic filters to run external command line programs that take attachments and message text as input.

2) attach a small text note to the message and search this notes for simple text strings.

3) let me store a message in multiple folders.

4) handle multiple accounts (5 at least) and be able to store at least 50000 messages.

5) provide good backup & restore functionality.

Anything like this available ?

EMail is my most important application and i want to switch my internet office computer to a MacMini but i can't if one of the 5 points is not there.

TheBat my windows email client supports all the 5 since 1998.