In order to send mail through the mail servers, the email client program must be configured to use SMTP Authentication. Common email clients include a tickbox in the account configuration with the label "This server requires authentication". That box should be ticked, otherwise look for a setting on the outgoing server settings that refers to authentication and set it to use the same username and password for the incoming server.
This error can also occur with some ISPs which block port 25 for SMTP. AOL is known to be such an ISP. If this happens, setting SMTP to port 587 will allow you to send email through us with AOL, for other ISPs we suggest contacting their technical support for advice or using their own outgoing SMTP server they would have provided you with.
You may also see this error shortly after a DNS change has been actioned. If a domain's DNS cache has not yet updated to reflect recent changes, you may face 550 errors. Waiting will resolve the matter in this instance.
Also check that you have the correct outgoing mail server settings. If you are using our mail server then this should be mail.yourdomain.co.uk