yellow triangle in black box

I have an IMac with OS X 10.8.2 with Microsoft Office for Mac 2011 Home and Business. In recent days, when I click on the send/receive to check for e-mails, a black box appears in the lower left hand corner of the screen. Inside there is a yellow triangle with a white exclamation mark inside. The text says, "5.1.2 or other punctuation after the recipient's email address. ww6sm102716361gb.2gsmtp This began after I forwarded an e-mail with graphics that I received. How do I remove this? Thank you.