Sending an email returns the error "553 sorry, that domain isn't in my list of allowed rcpthosts "

This means that you do not have authentication set up for sending mail. For example, if you are using Outlook, this must be set:

If you set everything up according to our instructions here in the help (we have instructions for other email clients as well), this error will not occur. First of all, make sure you have SMTP (sending) port 587 set