Bounced e-mail message

Hi,
Whenever I send an e-mail message to our entire club or the club officers, I keep getting an e-mail message regarding one of our club officer's e-mail address as follows: E-mail address was skipped because it has previously bounced too many times. Please visit
urldefense.proofpoint.com/v2/url?u=https...ABos-pbTDrGFUuFiA&e= and post a support request message for more information.
Do you know why his e-mail address is bouncing? What exactly does that mean?

We need your club number and either the first or last name of the officer affected

Thanks for the quick response.
I added my club number (3518) to my signature.
The affecter officer's full name is Nam Nguyen.

Nam's email address is invalid

Recipient Email Address: nnguyencpa@bellsouth.net
Diagnostic Code: 550 5.2.1 ... Addressee unknown,