I used Eudora via POP for thirteen years and relied on that functionality with the myriad folders I kept. But those single-client days are long gone. IMAP means that customers (including myself) can use an email app on their laptop, desktop, phone, tablet, and of course webmail. There is no reliable single-app filtering like in the old days. There is also no longer a single "sent" folder, as each app uses a different folder, like Sent, and Sent Messages, and Sent Items.
The only thing that's consistent between all of these is the connection to the SMTP server.
SmarterMail has Content Filtering built in, and should be easy to include in that code's scope.
Mik MullerMontague WebWorks