Microsoft 365 throttling diagnosis: separate connections from daily volume

Define the decision

A low campaign send count does not rule out throttling. Several applications or workers can open concurrent connections through the same mailbox. The relevant evidence is the timing and type of provider refusal.

Work through the procedure

1. Capture the response code, text and SMTP stage.
2. Inventory every application using the account.
3. Compare concurrent activity with the failure window.
4. Reduce the identified pressure and use bounded retries only for clearly unaccepted temporary failures.

Worked example

The following is a synthetic example for this procedure, not a customer result or performance benchmark.

Observed: 432 4.3.2 concurrent connections limit exceeded
Mailbox consumers: campaign worker plus another automation
Investigation: simultaneous submissions, not just daily total
Correction: coordinate concurrency across consumers

Read the result

Microsoft documents this specific concurrency response. Other failures need their own diagnosis; do not label every slow or rejected send as the same limit.

Check before moving on

1. Keep uncertain acceptance outcomes out of automatic resends.
2. Check ordinary mailbox traffic outside Zintara.

Limits and next action

The application’s configured daily cap cannot enforce limits across unrelated clients. Review the provider’s current limits and the actual shared usage before increasing capacity.

Source: Microsoft: SMTP submission concurrency and storage limits