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.<p>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.