Report SMTP server:
ERROR: 553 sorry, that domain isn't in my list of allowed senders (#5.7.1)
Connection interrupted!

Reason:
some SMTP servers can be e.g. part of offer the whole webhosting, and this is connected to certain domain (eventually its aliasy). These SMTP servers can have allowed to send emails only from domain (alias), to which is webhosting ordered. That means, if you will have setup identity (sender´s email) from domain, which you haven´t by your webhosting (for example freemails), and from domain, which you host, but you didn´t logon to its SMTP account, then sending of email needn´t be allowed.

Solution:
try to find out the right logon data to SMTP server (it means, if you will connect to SMTP without password needn´t mean it will send for a certainty) and or change sender´s email address.