I'm surprised no one has mentioned MH-E. I've used it continuously since the 80's. About 15 years ago it got multipart MIME support, which has improved every y

I'm surprised no one has mentioned MH-E. I've used it continuously since the 80's. About 15 years ago it got multipart MIME support, which has improved every year. I can handle attachments like a boss. I even wrote a filtering system that sits on top of it (a replacement for the inc program that sorts into different inboxes).