Cannot Add Contact to List Manually

Cannot Add Contact to List Manually

The sign-up form for our list is not working, so I have been adding users to it manually. I have one user that I cannot add. The system simply replies "0 Contacts Added", with no other explanation. I have checked our list of "Do Not Mail" contacts but cannot find this user in there.

I cannot post the user's email address here, but it doesn't have anything like "info" or "sales" in it that might be rejected. Please contact me to assist.