SmarterMail won't let me set up an alias that includes an address whose domain includes an underscore, such as user@g_mail.com. WHen I try to save the alias, it says it's not a valid address. Why? Is there a workaround?

As far as I know, you cannot register a domain name with an underscore. Therefore, setting up an alias with an underscore would not be allowed as that would be an invalid format.

Oops -- a user said there was an underscore in his address, and when I googled that domain I found another address, so I assumed they were OK. But they were both invalid.