Complete documentation can be found here:
If ASPMail is set up, but it is returning the error "550 - Not LocalHost Not a Gateway", the "FromAddress" variable must be set to a local address. For example, if your domain name is "abc.com," Your FromAddress must be YOURUSERNAME@abc.com.
The address must be a valid user as well, not an alias. This is necessary for our SMTP server to operate. This is one of our many SPAM blocks used to keep out access from the outside.