451 4.7.500: Server busy – try again later

Microsoft 365 / Outlook.comSMTP

The error usually looks like this:

451 4.7.500 Server busy. Please try again later from [x.x.x.x]. (S77719)

What it means

Microsoft is temporarily throttling email from the sending IP address. This is a temporary error and the sending server will retry automatically.

Common causes

  • The IP address is new, or has suddenly increased its sending volume, or has a poor reputation.

How to fix it

  1. Wait – messages are usually delivered on a later retry.
  2. If it persists, reduce your sending rate and check your IP and domain reputation.

Tools to help diagnose it

Email blacklist checker

Check whether a mail server IP address or domain is listed on common email blocklists

SPF, DKIM, and DMARC checker

Check your domain’s email authentication DNS records, count SPF lookups and diagnose deliverability issues

A free tool provided by Auth-Email

Auth-Email makes your Gmail, Outlook, or other OAuth 2.0 email account available to any IMAP, POP3, or SMTP email client — no app passwords or special configuration needed.

Read the docsTry Auth-Email →

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