|553: Sorry, that domain isn't in my list of allowed rcpthosts|
The reason for this error - "'553 sorry, that domain isn't in my list of allowed rcpthosts (#5.5.3 - chkuser)" - is that your mail application has not authenticated to our mail server.
In order to send mail through our server, your email program must authenticate. There are two ways to do that.
The first one is to enter the username and the password in the application so that it supplies them before sending. This can be configured in the options for the email account at the email program. This is the recommended method.
The second method is called "POP/IMAP before SMTP". Basically, if you check your mail first for incoming messages, the server will remember your address for some time, and you will be able to send through it during that period without giving your username and password. This is probably the method you are using at the moment. The error message would appear after you have not checked your mail for some time, and you try to send. In this case, you should either configure your mail program to authenticate to the server in order to send, or check your email before sending.