Noticed this happening for quite a while though I rarely use my Outlook.com email account. It is added to Mac Mail as an IMAP/SMTP account and all works fine apart from when I send emails they sometimes send twice. There's no error or any sign of it re-sending. I just compose an email, hit send and when I check the sent folder (both within Mac Mail and on the web client) there's two copies both with the same time stamp. Anyone else experiencing this?