Microsoft 365 / Outlook 452 4.5.3 (ATTR38): mixed regions.

The recipient list spans multiple Microsoft regions and cannot be processed in one transaction.

How to fix 452 4.5.3 (ATTR38).

Retrying or final
Retrying
A 4xx reply. Your server keeps the message queued and tries again.
Who is at fault
Sender
Your authentication, reputation, content, or sending pace.
Severity
WarningRegion Mismatch
Temporary452 4.5.3 (ATTR38)

Multi-tenant organisations often host users across regions. Microsoft requires you to send separate transactions per region.

Next steps

  1. 1Segment distribution lists so each message targets one Microsoft region.
  2. 2Review hybrid routing or smart hosts that override EOP’s decisions.
  3. 3Ask the recipient organisation to consolidate MX routing where possible.
  4. 4Leverage per-region connectors in Microsoft 365 if you manage the tenant.
Microsoft 365 / Outlook documentationEvery raw SMTP 452 reply

More Microsoft 365 / Outlook codes.

Full Microsoft 365 / Outlook playbook

Other Microsoft 365 / Outlook fixes

421 4.7.0Temporary system problemRetrying
451 4.4.4 (ATTR5)Temporary server errorRetrying
451 4.7.500 (S77719)Server busy (S77719)Retrying
451 4.4.62 (ATTR35)Wrong regionRetrying
451 4.7.650Sender blocked by EOPRetrying
550 5.4.1Recipient address rejectedFinal

In the glossary

Bounce (Soft)A soft bounce is when an email fails to deliver for a temporary reason, like a full inbox or a short outage at the recipient mail server. The address is still valid, and the email may be delivered if you try again later.
DeferralA deferral is when a receiving email server temporarily refuses a message but suggests trying again later.
ThrottlingThrottling is when inbox providers or your email platform slow or limit how many emails you can send in a set time. It acts like a speed limit so big sends do not overwhelm their systems or hurt your reputation.
GreylistingGreylisting is an anti-spam technique where a receiving server temporarily rejects emails from unknown senders to test if they are legitimate.

Browse all raw Outlook replies →

More references.

SMTP field manual.

Every SMTP reply code, with real bounces from mailbox providers and spam filters and who needs to act.

Catch bounce problems before your customers do.

Pre-send checks, send pacing, and live delivery reporting in one place. Free for 30 days.

Start free trialBook a demo