How to fix delivery delays

You got Sustained delivery delays are above 10% of sent mail. Receiving servers accepted the message later (or greylisted / deferred it). This measures delivery delay, not inbox vs spam.

What to change

Delays usually mean the far side is throttling you or retrying. Check, in order:

1. Volume spike — a sudden list blast after quiet weeks. Slow the send; don’t retry the same delayed cohort immediately.
2. List quality — high bounces/complaints in the same windows. Fix those first (bounce, complaints).
3. Authentication — open domain alerts on Settings → Deliverability. Failed DKIM/SPF/DMARC makes deferrals more likely.
4. Content — run the content checker on the campaign that was in-flight.
5. Dedicated IP warmup — if you use a dedicated IP, see How to fix a dedicated IP.

You cannot force Gmail or Microsoft to stop greylisting. You can stop looking like a bursty or unauthenticated sender.

After it recovers

The alert resolves when consecutive adequate windows stay below 10% delayed / sent.

Related

• Sender Health
• Campaign Delivery Score