I'd still say adding just one feature to an already working client is less work than writing one from the ground up. It's easy to think you can just grab a libr

I'd still say adding just one feature to an already working client is less work than writing one from the ground up. It's easy to think you can just grab a library to do IMAP. But what about attachments? Encodings? HTML mail? Storage? Search? Remember, all these will need a UI of some sort. And this is just the tip of the iceberg. Thunderbird 3.0 has custom tags. OpenOffice eats Thunderbird address books and can do mail merge. The one thing that's left is running a script when a message is received. That is actually a good idea for an extension.