If you manage to configure it to your taste, claws is a very capable mailer. It's a bit slow (I'm subscribed to many mailing lists, so I have many imap folders, some of them with more than 20000 messages, but claws copes with them fairly well albeit slowly). The built-in email application is a joke, I also tried modest but it would slow to a crawl then die while trying to download the headers for those 20000 messages. The biggest problem I have with claws, apart from the slowness, is that it isn't well integrated with the system (e.g. a mailto link will open the built-in mail program) but I gues it's maemo to blame since it makes almost impossible to get rid of the default mailer.