I keep getting an "Email Application Alert" which says 'Error sending: "(insert subject of email)"'

Weird thing is, it doesn't do it every time. I've been able to send other emails today, but it has given me the error 3 times now. It keeps trying to send, so the error keeps showing up over and over.

I am using an imap server for my Engine Hosting email accounts. Logins, ports, etc are all correct.