#200 Allow filtering by email account

[Chris Handley]

It would be extremely useful if I could specify filters that matched (or didn't match) particular accounts. Filtering by email address is not always sufficient, due to auto-forwarding, mailing lists, etc.

This would allow me to prevent my filters from moving IMAP emails (where I obviously want them to stay in their existing folders), and so allow me to use both POP3 *and* IMAP simultaneously for one email address.

It would probably have all sorts of other uses too!