I wrote a basic one, and I think the answer is that there are just so many moving parts (many of which I didn't attempt). - mail transfer and delivery (IMAP/PO

I wrote a basic one, and I think the answer is that there are just so many moving parts (many of which I didn't attempt). - mail transfer and delivery (IMAP/POP3, SMTP) - organization, folders, tags, database, search, indexing, etc. Performance becomes a big issue. - Email threading (conversations) is a whole thing. - Email file format, MIME standards, parsing emails is a WHOLE thing. (What is an rfc822-formatted email and why I am getting them?) - For non-text email, need an entire HTML rendering engine. - The graphical user interface is not trivial, should be intelligently multithreaded. - More features: spam filtering, encryption/digital signatures. Vacation replies. Address book. Calendar integration is a whole (optional) thing. Todo-lists. - For some reason, Thunderbird has a live chat system. It also seems to be the kind of thing where the last 10-20% is a lot harder than the first 80%.