The server response was: sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)
I checked the settings in Plesk and the mail server is set to allow relaying the messages for SMTP authentication.
My ISP says the following thing in their FAQ:
When I try to send mail using SMTP through my mail client I get the following error: 553 sorry, that domain isn\'t in my list of allowed rcpthosts (#5.7.1)
By default the mail relay is set to closed within PSA. If the mail relay is set to closed and you plan to allow SMTP relaying, the mail relay should be set to require SMTP or POP authorization. Note that if you set the mail relay to require SMTP authorization you will need to enable SMTP authentication within your mail client (Outlook, Eudora, etc..)