Can not send email using Outlook (Error: ‘550 not local host, not a gateway’)

If you are receiving an error similar to this when sending email using Outlook:

“The message could not be sent because one of the recipients was rejected by the server. Server Response: ‘550 not localhost, not a gateway’, Port: 25, Secure (SSL): No, Server Error: 550, Error Number: 0x800CCC79

  1. Open your Outlook settings (Tools ->Accounts)
  2. Go to View/Change email account for the email account you are having problems with.
  3. Check that the SMTP Server is set to
  4. Go into the “More settings”, “Outgoing Server” tab and make sure that the option “My server requires authentication” for the Outgoing server is checked ON.
  5. Save and close all windows.
  6. Try again.

If the problem continues then the problem is that your Internet Service Provider is blocking port 25 on the outgoing mail server. ISPs do this to prevent spam on their network. When this happens you will need to use the mail server that they provide you instead of your own. The mail server is generally, however, it could be different so please contact your ISP to get the details.

If you do not want to use your ISPs server for SMTP, and the ISP is blocking port 25, you can switch the port to 587 for your outgoing mail and use for the SMTP server name.

This article is specific to users of 3dcart Shopping Cart Software.