What Does 550 5.7.233 Mean, and How Do I Fix It?

What the bounce says

The 550 5.7.233 error is not about the recipient or your reputation. Exchange Online stopped the message before it left your tenant, because the tenant as a whole reached its daily quota of external recipients. Microsoft's troubleshooting page puts it plainly: "All tenant users sending to external recipients are blocked."

5.7.233 has two neighbors from the same family of limits:

MC1454397 words 5.7.233 more briefly: "Message blocked due to tenant external recipient limit." The email bounce decoder tells the variants apart.

How Microsoft counts the limit

The quota comes from the number of email licenses in the tenant, per the Exchange Team Blog (updated in August 2026): 500 × (non-trial email licenses^0.7) + 9,500 external recipients in a 24-hour sliding window.

The two right-hand columns come from MC1454397 (published August 14, updated September 29, 2026): a tenant younger than 31 days gets 10% of its quota, 31–60 days 25%, then 100%. Trial tenants get 500 external recipients a day regardless of license count, down from 5,000.

What counts: every recipient whose domain is not an accepted domain in your tenant, including mailboxes in other Microsoft 365 tenants. Repeat sends are not deduplicated: "If I send 1,000 messages to the same external recipient in a day," the blog answers, "It counts as 1,000." Out-of-office replies, NDRs and journaling do not count.

Why cold email tenants hit it first

A cold email setup on Microsoft 365 spends the quota three ways at once: first touches, every follow-up, and warmup. Warmup mail goes to mailboxes in other organizations, so each warmup email is an external recipient too.

Take a tenant with 100 licensed mailboxes. After day 60 it has 22,059 external recipients a day, about 220 per mailbox. In its first 30 days it has about 2,206, or 22 per mailbox for prospects, follow-ups and warmup together. A trial tenant with the same 100 mailboxes has 500 a day: 5 per mailbox. Once the cap is spent, the block also stops the tenant's ordinary mail, including replies to warm leads.

For the full list of per-mailbox and tenant limits, see Microsoft 365 sending limits.

How to confirm it

1. Open the report. Exchange admin center > Reports > Mail flow > Tenant Outbound External Recipients Rate shows the quota, how much is used and how many recipients were blocked.

2. Check enforcement. The report and the Get-LimitsEnforcementStatus cmdlet in Exchange Online PowerShell show EnforcementEnabled. Verdict: Block with enforcement on means mail is being stopped now.

3. Check the tenant's age. Under 60 days, the threshold is 10% or 25% of the formula.

How to fix it

1. Pause the campaigns on that tenant. Every retry and every warmup email still counts against the window.

2. Let the window roll. The block clears when the external count for the last 24 hours drops below the quota. Microsoft says this takes "minutes or up to 24 hours", depending on when the mail went out.

3. Don't raise the outbound spam policy. Its limits are separate settings. In a Microsoft Q&A thread on this code (February 2026), the accepted answer notes that changing the policy did not lift TERRL.

4. Plan from the real cap. Divide the current threshold by the mailboxes, then subtract warmup and follow-ups.

5. Use what Microsoft recommends for more volume. Microsoft points tenants near the cap to more email licenses or a dedicated high-volume or non-Microsoft bulk email service.

The autumn changes are covered in Microsoft 365 new-tenant limits for cold email.

What we recommend at Outreach2day

Our mailboxes are standard SMTP/IMAP on our own infrastructure, outside Microsoft 365, so Exchange Online's tenant quota does not apply to them. We recommend up to 25 cold emails per mailbox a day, with warmup on top of that, after 14 days of warmup. If your outreach runs on Microsoft 365 tenants and 5.7.233 keeps coming back, see moving cold email off Microsoft 365.