How to Stop Trying to Send E-mail Message That Is Too Large?

I'm seeing repeated error messages telling me that Gmail (I know--this is not a Gmail forum) can't send an e-mail message that is too large, and I'd be grateful for suggestions from more tech-savvy users about how to make them stop.

I use a MacBook Pro, running OS X Yosemite v.10.10. I use Microsoft Outlook for Mac as part of Office for Mac 2011. I have Outlook for Mac synced with a Gmail account. A couple of days ago, I tried to send an e-mail to someone with several files attached, one of which was a large video file. The message would not send, and I figured out that it was too large and then deleted it from my Outlook Outgoing Mail folder. Now, when I launch Outlook, I no longer see that folder, but I keep getting these error messages on my Desktop. They show up in a little dark-gray window that says Gmail at the top and that has a yellow triangle with an exclamation point inside it, on the left side of the window. The text in the window says, “[ALERT] Message too large. http://support.google.com/mail/bin/answer.py?answer=8770”.