How to re-add "do not mail" email to contact list

Hi,

We have a small list of customers that have been added to our list by signing up in person at our place of business. We sent out a mailing announcing an event and one of our customers put us in his spam folder accidentally. I recognized his email address as our only complaint from our latest campaign, so I called him and this is what he told me. He told me he then removed the email from the spam folder but apparently the damage is already done.

Of course, he wants to be added back to the list so he can be notified of our events but I can't add him back. Is it possible to do so?

Gail