Overview

You try to add a new valid email address as a contact but ACM validation fails with the error "Please enter a valid email address":

Solution

Current validation in ACM UI doesn't allow adding emails with nonstandard domains.

But there is a workaround. You can add the contact through the EmailLabs interface.

1. From within ACM, click on Launch EmailLabs from the menu on the left-hand side. This will open EmailLabs interface.
2. Select your mailing list.
3. Click Add Member Record
   
4. Add the member
   

You need to be sure that you adding an existing valid email. Invalid emails will not pass validation in EmailLabs.