From Michael Kwasnicki:
MailWidget checks your Apple Mail accounts in the background and notifies you, when new mail has arrived. Unlike for other widgets Apple Mail does not need to be running all the time. It is able to check your POP, IMAP, Exchange 2007 and MobileMe accounts. Moreover MailWidget knows which mails from your POP account have been received by Apple Mail. This is especially interesting for those, who do not have an IMAP account and so do not remove the mails instantly from the server in order to be able to access them via WebMail on travels. In such a case other widgets would tell you something like: "There are 69 mails on the server". Only MailWidget will tell you which of those are new and have not been already received by Apple Mail.
What's new in this version:
Resolved issue iCloud accounts not being shown in Mavericks and Yosemite.