Outlook is no longer sending email from my smtp server

Outlook is most likely no longer using the Rackspace SMTP server and is instead trying to relay mail through Outlook.com/Microsoft 365, which then rejects or reroutes messages.

Relay errors (550/553/relay prohibited) occur when the SMTP server does not recognize the sender as authorized to send for that domain. When Outlook is configured to use a different SMTP server than before, or when authentication/ports change, the server can reject outbound mail.

Use these checks and fixes:

1. Verify Outlook is online and connected

• In Outlook, ensure Work Offline is not enabled.
• Confirm other internet services work.

2. Confirm the account’s SMTP server is Rackspace

• Open Account Settings for the business email account.
• Under Outgoing server (SMTP), ensure the server name is the Rackspace SMTP host, not an Outlook.com or Microsoft 365 server.
• If the ISP or network has changed, confirm the current network/firewall allows connections to the Rackspace SMTP server and port. If blocked, request an alternative SMTP server or port from Rackspace and enter it in Outgoing server.

3. Turn on SMTP authentication for the account Some POP/IMAP accounts require authentication on the outgoing server:

• Go to Tools (or File → Account Settings) → Accounts.
• Select the business email account.
• Under Outgoing server, choose More Options (or similar) and enable authentication.
• Enter the correct username and password for the Rackspace account.

4. Verify credentials and special settings

• In the account settings, confirm the email address, username, and password are correct.
• If mail still fails, check with Rackspace for required settings such as SSL/TLS and non-default ports, and configure those in Outlook.

5. Check for relay restrictions / spoofing protections If Outlook is trying to send with a From/reply address that doesn’t match the domain authorized by the current SMTP connection, the server can reject it with 550/553 or relay-prohibited errors. Ensure:

• The From / reply address domain matches the domain that Rackspace expects for that SMTP login.
• The return address is not set to an Outlook.com address when using Rackspace SMTP, or vice versa.

If Outlook continues to route via Outlook.com instead of Rackspace, remove and re-add the account using the full Rackspace POP/IMAP + SMTP settings so Outlook is forced to use the correct outgoing server.

References:

• I receive a 550, 553, or relay-prohibited error when sending email messages
• I can't send or receive messages with Outlook for Mac