Repetitive sending error in Outlook

Hey Everyone, rank newbie here, finally made the move to a Mac, and wonder why I waited so long!

I'm stumbling my way through most everything, just have a quirk with Outlook.
I clicked "Reply All" to an email that was addressed to "undisclosed recipients", that was about a month ago. Outlook continually tries to re-send this reply, and produces an error,
5.5.0 <undisclosed-recipients:;> invalid address 'undisclosed-recipients:;'
error code: - 17099

Anyone tell me how I can stop this from sending?

Thanks in advance!

Welcome to the forum, and to the Mac world.

I found this with a quick search: http://www.stellaroutlooktools.com/blog/fixing-error-code-17099-in-ms-outlook-for-mac-2011/

I have no idea if it works, just did a search.