Here’s a clean, practical workflow I use to connect a custom domain, add email aliases that show in the From list, and prepare a mailbox for SMTP client submission in Microsoft 365.

1) Add a Custom Domain

  1. Sign in at outlook.office.com, open the app launcher (nine dots), and choose Admin.
  2. Go to Settings → Domains and click Add domain.
  3. Enter your domain and complete verification by adding the requested TXT record at your DNS host.
  4. Apply the DNS records Microsoft suggests for email (typically MX, CNAME Autodiscover, and TXT SPF).
  5. (Optional) Set the domain as default if you want new users created under it.

Notes: Publish only one SPF record; merge with existing if needed (e.g., v=spf1 include:spf.protection.outlook.com -all). DNS propagation may take time depending on your TTL.

2) Add Email Aliases (and Send From Them)

  1. In the Admin center, go to Users → Active users.
  2. Select your account → Manage username and email.
  3. Click Add alias, choose your custom domain, enter the address, and save.
  4. (Optional) Set the desired address as Primary.

Important: It can take several hours—sometimes up to a day—before new aliases appear in the From picker in Outlook. If you still don’t see them after propagation, check the Exchange admin setting to allow send from aliases.

3) Prepare the Account for SMTP Sending

For apps and devices that send mail via SMTP client submission:

  1. Ensure Authenticated SMTP is allowed for your organization and enabled on the mailbox (Exchange admin settings).
  2. Choose your authentication approach:
    • Preferred: Use a dedicated mailbox for SMTP and keep MFA on for user accounts. If supported, use an app password for older SMTP clients.
    • Alternative (less secure): Disable per-user MFA for the specific SMTP-sending account: Users → Active users → Multi-factor authentication → select user → Disable (if enabled).
  3. SMTP settings:
    • Server: smtp.office365.com
    • Port: 587 with STARTTLS/TLS
    • Username: full mailbox address (e.g., [email protected])
    • Password: mailbox password or app password
    • From: mailbox address or one of its aliases (send-as from another mailbox requires permission)

Security tip: If SMTP is required, prefer a separate, restricted mailbox and monitor usage. Consider Microsoft Graph or other modern APIs for application mail when possible.

Troubleshooting

  • Alias missing in From: Wait for propagation; sign out/in to Outlook web or restart Outlook desktop.
  • SPF errors: Keep a single SPF record and include Microsoft 365.
  • SMTP blocked: Confirm Authenticated SMTP is allowed org-wide and enabled on the mailbox.
  • Domain won’t verify: Recheck TXT name/value and allow time for DNS TTL.