Mail Server error 550 No such user here & How to emove it
This error message is reported by the e-mail server to your email client such as Outlook or Outlook Express because the check box labeled "My Server Requires Authentication" is not checked. 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.
Resolution:To resolve this issue check the checkbox next to "My Server Requires Authentication".
Open Outlook > Click Tools > E-Mail Accounts > Select the email account > Click Change > Click More Settings > Click the Outgoing Server Tab > Check the checkbox "My outgoing server (SMTP) requires authentication" > Click OK > Next > Finish
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.