421 4.7.500 — Server busy, try again later (Outlook.com)
Retrying later can work — at a lower rate, not the same one.
The reply
421 4.7.500 Server busy, please try again later.A temporary throttle. Microsoft is limiting how much it will take from this IP right now. Retrying is correct; retrying at the same rate is not, and is the usual route from a throttle to a block.
What it means
Despite the wording, this is rarely about Microsoft's servers being busy. It is a rate limit applied to your IP, and how tight it is depends on your reputation there. A new or low-reputation IP gets throttled at volumes an established one would not notice. The right response is to reduce concurrency and spread the send.
Why it happens
- Too many simultaneous connections from one IP.
- A volume spike from an IP with little history at Microsoft.
- Low IP reputation, which tightens the limit.
- An IP still warming up that is being pushed at full volume.
What to do
- Reduce concurrent connections to Microsoft and lengthen the gap between messages.
- Back off exponentially instead of retrying on a fixed timer.
- Check SNDS for the complaint rate and trap hits on the IP — a tight throttle usually reflects reputation.
- Warm new IPs gradually; a cold IP at full rate meets this immediately.
- Spread large sends across hours rather than minutes.
Questions
Will the mail eventually go through?
Usually yes, provided you slow down and your retry window is long enough. If it keeps deferring for hours at reduced rate, the cause is reputation rather than throughput.