Problem:

When adding a forwarding address in Smartermail, I get the following error:

Forwards to the following address(es) are not allowed:
someaddress@somedomain.com (aol.com, comcast, or verizon)

Answer: 

3Essentials does not allow forwarding to some domains.  Details provided here: 

http://knowledge.3essentials.com/web-hosting/article/848/3Essentials-Email-Forwarding-Policy.html