If Apple Mail is causing you trouble, it might be that you need to Rebuild your mailbox. In Mail, choose ‘Rebuild’ from the Mailbox menu. The operation might take some time to complete, but can solve performance problems and reduce the size of the mailbox database. If your mail uses IMAP, some of the settings might be lost after a rebuild. It should be pointed out that this should only be done if you’re having problems, rather than as a preventative or maintenance method.