Outlook keeps changing server settings and it is almost impossible to repair the changes

The behavior described matches known POP/SMTP configuration and relay issues in Outlook rather than a permanent change made by Microsoft.

Key points from the scenario:

• POP/SMTP account in classic Outlook for Windows.
• SMTP server name keeps ending up different from the POP server name (e.g., mail.provider.com.au vs mx3.provider.com.au).
• Error from SMTP: 451 4.7.1 Service unavailable - try again later.
• Webmail works, so the mailbox and provider are fine.

What is happening

1. Outlook’s “Repair” and auto-discover logic can change server names and ports based on what it detects from the provider or DNS. For POP/IMAP accounts, Outlook may try to “optimize” settings and pick a different SMTP host (such as an mx host) if it thinks that’s correct.
2. If the ISP or provider has restrictions on relaying or specific SMTP hosts, using the wrong SMTP server (like mx3...) can cause errors such as 451/4.7.1 or other SMTP failures.
3. Some ISPs and providers also enforce relay rules (e.g., only allowing certain SMTP servers or requiring authentication). If Outlook switches to a host that is not meant for client submission, messages can be blocked.

How to stop it and stabilize settings

1. Manually verify and lock in the correct POP/SMTP settings:
   • Open Control Panel → Mail → Email Accounts.
   • Select the POP account → Change.
   • Confirm the Incoming mail server and Outgoing mail server (SMTP) exactly match the values provided by the email provider (for example, both using mail.provider.com.au if that is what the provider specifies).
   • Under Logon Information, confirm the correct username and password.
   • Select More Settings → Advanced and verify ports and SSL/TLS options match the provider’s documented POP/SMTP settings.
   • Save and close without running automatic “Repair” again unless necessary.
2. Avoid using “Repair” if it keeps rewriting the SMTP server:
   • If the account works after manual configuration, avoid running the automatic Repair wizard, as it may re-detect and reapply different server names.
3. Check for relay-related restrictions:
   • If using the account from different networks (home, work, hotel, etc.), some ISPs treat this as relaying and may block messages when the SMTP server does not match the current connection’s provider.
   • In such cases, configure Outlook to use the SMTP server of the current connection (for example, the work SMTP server when at work) instead of the home ISP’s SMTP server. This is a standard requirement when ISPs restrict relaying.
4. If the provider requires SMTP authentication or POP-before-SMTP:
   • In Account Settings → More Settings → Outgoing Server, enable “My outgoing server (SMTP) requires authentication” and use the same credentials as the incoming server if the provider requires it.
   • This aligns with common ISP restrictions on relay and helps avoid 550/553/relay-prohibited-type errors.

Is this related to recent Microsoft updates?

• The context provided does not identify a specific recent Outlook update that automatically changes POP/SMTP servers to mx3 or similar. The behavior is consistent with Outlook’s auto-configuration and ISP relay restrictions rather than a confirmed bug.
• There is a known pattern where Outlook’s automatic configuration and Repair routines adjust server names and ports based on provider metadata. If those values are not what the provider expects for client SMTP, errors like 451 can result.

What Microsoft is doing / can do

• Official guidance focuses on:
  • Verifying and manually correcting POP/SMTP settings via Control Panel → Mail → Account Settings.
  • Ensuring the correct incoming/outgoing server names, ports, SSL/TLS, and authentication settings are used.
  • Handling relay restrictions by using the SMTP server appropriate to the current network connection.
• If the problem persists even with correct settings and without using Repair, the next step is to work with the email provider and Microsoft support to check for:
  • Provider-side relay restrictions or blocks.
  • Any account-specific issues or backend changes affecting SMTP.

In practice, the most effective mitigation is to manually configure the correct POP/SMTP settings as documented by the provider, avoid the automatic Repair if it keeps changing the SMTP host, and ensure relay rules and SMTP authentication requirements from the ISP are followed.

References:

• Change or update email account settings in Outlook for Windows
• I receive a 550, 553, or relay-prohibited error when sending email messages
• I can't send or receive messages in Outlook