Detailed release notes are included with the latest update. Most of the changes are minor features or bug fixes. Note the new application icon and the new widescreen layout (previously described in a blog post).
MailMate has entered the final phase before the 1.0 release. Please report any serious issues with the current version of MailMate. Do not expect any new features until after the 1.0 release where development will continue with regular updates (similar to the 0.9.x updates).