Trying to add a "Valid FROM Email address" and it won't accept it

Hi!  My Weekplan acct is registered under my gmail, but I'd like to add my work email address so I can add tasks from that email.  When I went to try to add it, I get this message "Email address could not be added".  I was able to add a different personal email address (from AOL.com), but not my job one.  Is there a reason why that would happen or a way to resolve?