My MBP has stopped sending and receiving email.

I haven't received any email on my MBP since April 10. I wrote an email this morning and it is still in my outbox.Emails are coming in to my iPhone 6+ and my iPad. The emails are showing on my iCloud app online. Other apps such as iCal are updating normally. I have internet connection. i.e.Safari works fine. I haven't changed any settings in accounts. All of them seem to be online, Any ideas?