Part 1 · 1.4
451 4.7.500 Server busy: Microsoft is throttling your mail
Are you emailing an Outlook.com, Hotmail or Live.com address, or a company that uses Microsoft 365, and getting a message with 451 4.7.500? Then your email hasn't arrived yet, but it hasn't failed either. This is a temporary error: the code starts with a 4.
What the message looks like
451 4.7.500 Server busy. Please try again later from [203.0.113.25]. (AS843)
You receive this as a delay notification, often with Action: delayed. Microsoft also uses Access denied, please try again later for the same range of codes.
What does it mean?
Microsoft's server isn't actually busy. Microsoft is temporarily throttling the IP address of the sending server so it can assess the traffic, for example when a sudden increase in email comes from it. Microsoft lifts the restriction automatically once the email turns out to be fine. Microsoft has no manual whitelist: it's always an automated system.
What happens to your email?
Our server keeps the email and tries again by itself. Usually the email arrives after one or more attempts, sometimes only after a few hours.
How to deal with it
- Don't send the email again. Otherwise it will arrive twice later on, and more email tends to make the throttling worse.
- Wait. With E-mail Tracking in DirectAdmin, you can see whether the email has been delivered in the meantime (see E-mail Tracking: where has my email gone?).
- Make sure the SPF, DKIM and DMARC records for your domain are set up correctly. Check them with the Email Health Check.
- Don't send newsletters from your regular mailbox. Use a newsletter service instead.
Please note: Are you getting 550 5.7.606 or banned sending IP? That's not throttling but a block. See Error code 550: which variant do you have and what should you do?.
When should you contact us?
Does an email really need to arrive right away, or does this keep happening for days? Then send us the delay notification.
Ask the AI assistant in My WebOké first, or create a ticket via Support centre and then Submit a ticket. Include:
- the full delay notification
- the recipient's address
- the date and time you sent the email
Never send your password.
Read more: Email to Outlook, Hotmail or T-Online delayed or rejected and, in the Email in Order course, Difficult recipients: Microsoft and T-Online.