**New Information**
I, too, use ipowerweb as my isp. Ran up against the exact same problem of a 553 Error.
Then I went to ipowerweb's online helpcenter. Usually the help provided isn't much help there, but in this case it clued me into the real problem.
The message from smtp servers, "553: Sorry, that domain isn't in my list of allowed rcpthosts" is not much help in diagnosing the problem I was having. Here's what ipowerweb said:
This error occurs when a user tries to send mail and the server blocks the mail from being sent. There a number of reasons as to why this occurs and we will discuss a few of them below.
1) iPower email is configured to authenticate using POP rather than SMTP. This means that our server verifies who you are based on the incoming mail server, your username, and password. When you check incoming mail, the server opens a gateway of 20 mins where it will allow you to send your mail out to your intended recipients. Should the 20 minutes pass, the server no longer recognizes who you are and will need to verify security again using the incoming mail server.
The issue starts when email clients (such as Outlook) have an option set to send messages immediately. When you send a message immediately, your email program skips the authentication (verification) process and tries to send without receiving. If this happens after the 20 minute gateway closes, the server gets stuck because it has no idea which user is sending this email. It then stops, and says (not literally), "I do not know you; therefore, I will not let your mail through".
Basically, "553, Sorry, that domain isn't in my list of allowed rcpthosts" means that the server is not recognizing you.
So what are some ways to avoid this? We have two easy methods (A and B) to stop this from happening in Outlook and Outlook Express....
Well they did not address the problem we have with SwiftMail, PHPmailer, and the like. "How do we do a POP befor SMTP," I wondered. Then I found the Swift documentation that adds this step. See the bottom of page
http://www.swiftmailer.org/wikidocs/v3/smtpauth for how to do pop before smtp.
-Tony