FAQ  > Mail  > Mail management > Where does the following error come from : 553 sorry, that domain isn’t in my list of allowed rcpthosts (#5.7.1) ?

This message is a result of anti spam protections installed. To be able to send email using our SMTP servers, you have to first consult our POP accounts, which allows you to be identified. With Outlook, carry out the operation send / receive twice, at the second go the mails will be sent.