Outlook 2011 is sending blank emails to my contacts

I'm working on a new Mac Yosemite, using Office 365 Home, Outlook 2011 for Mac.

The blank emails aren't showing in the sent box, but people keep replying asking if I meant to send something.

The emails seem to go out either at the same time as other real messages or when someone is replying to real email messages I sent them.

This is really embarassing as this is my business email. I don't want to delete and re-create my account as I don't want to lose anything.

Are there other solutions I can try?

Please help!