Microsoft 365 Outlook Email With Custom Domain - Intermittently Working

Question

I have recently set up my email with my custom domain name by adding the relevant MX and TXT into c-panel, however occasionally my emails won't send.

I have a screenshot of the records (attached) - I'm not sure if I've set something up slightly wrong when connecting my domain? Could someone please help - this isn't my forté at all.

This seems to be more of an issue when using the Outlook mobile app, if that makes a difference?

I've tried to speak to Microsoft support, but I apparently don't have admin privileges, and I can't log in to the generic @onmicrosoft account which I assume has this access, because Authenticator isn't working / sending a code to my registered device! It's a complete nightmare.

Error message is: "Delivery has failed to these recipients or groups, your message wasn't delivered because the recipient's email provider rejected it"

Answer 1

Hi @James Frost ,

The error message suggests your emails are being rejected by the recipient's server, often due to authentication or DNS misconfigurations. Here's what to check:

1.Verify MX Records (Critical for Receiving/Sending)

• Correct MX Records for Microsoft 365:   Priority 0: <yourdomain>.mail.protection.outlook.com Ensure there are no other MX records (e.g., from your hosting provider or cPanel email). Extra MX records can cause conflicts.
• Check in cPanel: Go to DNS Zone Editor and confirm:

Only Microsoft 365 MX records exist (with mail.protection.outlook.com).

Delete any MX records pointing to your hosting provider (e.g., mail.yourdomain.com).

 2. SPF Record (Prevents Spoofing/Rejection)

• A single SPF record starting with v=spf1 include:spf.protection.outlook.com -all.
• Example:   v=spf1 include:spf.protection.outlook.com ~all (less strict) or -all (strict)).

  3. DKIM & DMARC (Authentication)

•  Microsoft 365 requires you to generate DKIM keys in the admin center and add them as CNAME records in cPanel. If missing, emails may fail authentication.
• Check your Microsoft 365 Admin Center > Domains > DNS Records for DKIM settings.
• Add a TXT record for _dmarc.yourdomain.com with:  

      v=DMARC1; p=quarantine; pct=100; rua=mailto:******@yourdomain.com 4. Outlook Mobile App Issues

• Ensure the app uses Modern Authentication (OAuth2) instead of "Basic" credentials. Go to Outlook app settings > Account > Advanced > Authentication Method.
• Server Settings: Outgoing (SMTP) server should be smtp.office365.com on port 587 with TLS/SSL encryption.
• Reinstall the App: Sometimes cached settings cause issues. Delete and re-add the account.

5. Microsoft 365 Admin Access

• You’ll need admin access to verify DNS settings and licenses. Try these steps:
• Use the global admin account (******@yourdomain.onmicrosoft.com) to log in.
• If Authenticator isn’t working, reset the admin password using an alternate email or phone number.
• Contact Microsoft Support again and provide proof of domain ownership (e.g., DNS records) to regain access.
• Wait 48 Hours**: DNS changes can take time to propagate.

If issues persist, share a redacted screenshot of your DNS records (hide sensitive info), and we can review them here.

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