How can you appeal Microsoft throttling?

You're getting 421 errors from Outlook and your emails are crawling through at a fraction of normal speed. That's throttling. It's annoying, but it's different from being blocked outright, and knowing that difference is step one.

421 vs 550: what's actually happening

A 421 error means Microsoft is slowing you down, not turning you away. Your mail will eventually get through if you back off and retry properly. A 550 means the door is shut. The appeal process is different for each, so don't skip this diagnosis step.

First: fix what's triggering it

Before you submit anything to Microsoft, tighten up your sending health. An appeal lands much better when you can show you've already made changes. Check these off first:

• SPF, DKIM, and DMARC are all published and passing (you can verify with our free SPF checker and DKIM lookup)
• Your sending volume is ramped gradually, not sent in large spikes
• Your retry logic respects the 421 signal and backs off instead of hammering
• Complaint rates are below Microsoft's thresholds (this matters enormously to their filters)
• Hard bounces are suppressed and your list is reasonably clean

Then use the Sender Support form

Microsoft's postmaster portal has a sender support request form. It's the official channel, and it's worth using when throttling persists after you've already addressed the technical issues. When you submit, include:

• Your sending IPs and the domains you're mailing from
• A plain-English description of your email program (who you send to, why, how they signed up)
• What specific changes you've made since the throttling started
• Your opt-in process and how you handle complaints and unsubscribes

Response times range from a few days to a couple of weeks. Microsoft rarely tells you exactly why filtering happened (that's just their policy), so don't expect a detailed explanation back. What you're really doing is demonstrating that you're a legitimate sender who takes deliverability seriously.

If your throttling started around the same time as a big campaign or a list import, mention that context. It helps them connect the dots and shows you understand your own program.

Still stuck after all that? Drop into our SOS hotline, it's free, and we can help you figure out whether it's a reputation issue, a list problem, or something else entirely before you send another appeal into the void.