I get the following error when sending emails.
Server error: '550 No such user here'
This is caused if you attempt to send email without using SMTP authentication.
The reason is that because you have not authenticated, the mail server then treats your email client just like any other sending mail server and so expects the recipient email address to be local rather than outside of our network. By authenticating with the server it knows you are authorised to send mail outside of our network and will instead forward the mail to the correct mail server for the domain.