Emails in Outbox won't send (400) Bad Request

Emails are stuck in outbox, some are sent, some won’t (at random, it looks like). I have a Gmail and a Hotmail account, Hotmail is the only one with this problem: outlook server problem?

@ eM client, this is a problem you have incurred many times in the past. Will this ever be resolved?  ![]( 1 inline.png?1456400224)

Hi Nicholas,
Microsoft is making many changes for its outlook/hotmail accounts, so if you have the account set up with AirSync protocol, there could be plenty of issues, since this mail protocol is being discontinued for MS servers. I suggest setting up the account again manually as IMAP.

However, if this exact problem continues or if you already have the hotmail account set up as IMAP, please copy the content of the LOG next time (third tab in the same Operations window as the Errors are).