Using the webmail interface, you are able to send emails to any domain without any bounce backs. But when sending emails using a PC email clinet such as Outlook, you keep getting bounce back emails saying: 550 <firstname.lastname@example.org > No such user here.
If you get the 550 No such user here bounce message for all emails sent, and you can send your emails using the webmail interface. Then your problem is that you have your PC email client not configured correctly. You need to make sure your from email address in your email client matches the email address used for authentication.
IE: Your true email address is: email@example.com. So you would have firstname.lastname@example.org and your password setup to authenticate to outgoing mail servers. But your from email address is configured as: email@example.com Having your email account configured like this will cause all sent emails to be rejected (bounced back) to you. This is because the mail server is doing a security check on all emails sent through the server. To resolve this problem, change your from email address from firstname.lastname@example.org to email@example.com. Then you will be able to send emails again.