Microsoft 365 550 5.7.236: daily limit for external recipients from onmicrosoft.com domains

Short answer: You are sending from your tenant's free onmicrosoft.com address, and Microsoft allows those only 100 external recipients per organisation in 24 hours. Fix: add your own domain to Microsoft 365, make it the default and every user's primary address, and set up SPF, DKIM and DMARC.

The bounce looks like this
550 5.7.236 Your message can't be sent because your tenant has exceeded its daily limit for sending email to external recipients from your tenant's onmicrosoft.com domains. For more information see https://aka.ms/EXONdrs.

What it means

Every Microsoft 365 organisation gets a free address like yourcompany.onmicrosoft.com. Because spammers abuse these, Microsoft throttles outbound mail from them: at most 100 external recipients per organisation in a rolling 24 hours, counted after distribution groups are expanded. Mail between colleagues and incoming mail are not affected. The limit is fully rolled out (2026), so organisations that never added their own domain now hit it after a few messages, and waiting only frees up the next 100.

How to fix it

  1. Check the From address of the bounced message. If it ends in .onmicrosoft.com, that's the cause. Admins can run a message trace in the Exchange admin center to see which mailboxes, printers and apps still send from it.
  2. Add a domain you own: Microsoft 365 admin center → Settings → Domains → + Add domain, verify it with the TXT record shown, then add the DNS records it lists. No domain yet? Buy one from any registrar first.
  3. Make it the default: on the same Domains page select your domain and choose Set as default. This only changes new accounts.
  4. Move existing users: Users → Active users → the user → Manage username and email, and pick your domain. Do the same for shared mailboxes, and change the sender address and username on printers and apps.
  5. Set up authentication for the new domain so mail reaches the inbox: SPF with include:spf.protection.outlook.com, DKIM turned on in Microsoft 365, and a DMARC record. Follow the Microsoft 365 guide and the DMARC generator, then check the domain below.
  6. Microsoft's documented fix is a custom domain. Until users send from it, external mail from the onmicrosoft.com address keeps bouncing after every 100 recipients.

Check your domain now

After the fix, send a test email to see SPF, DKIM and DMARC pass on a real message, then keep an eye on it: free DMARC monitoring shows every service that sends mail as your domain and which ones fail, in plain English.

Rather not touch DNS? I set up SPF, DKIM and DMARC without breaking your other senders, with a before/after report.

Get it fixed on Fiverr

Prefer to do it yourself? Get the step-by-step Fix Kit ($15) – 28-page guide, checklist, 20+ providers.

Other bounce errors