451 4.7.500 Access denied, please try again later
Microsoft 365 is temporarily throttling the sending IP
Seen in Microsoft 365, Exchange Online Protection (inbound)
Exchange Online is greylisting the sending IP (codes 4.7.500 to 4.7.699, with an ASxxx tag) because its volume jumped well above its usual pattern.
Common causes
- A new sending IP or a sudden spike in volume to Microsoft 365 recipients.
- Mail relayed from your own server through Microsoft 365 without an inbound connector.
- Sending real volume from an onmicrosoft.com domain, which Microsoft limits.
How to fix it
- Let your server keep retrying; the throttling usually lifts as the IP builds a history.
- If you relay through Microsoft 365, set up an inbound connector for your server.
- Send from a verified custom domain and ramp new IPs up gradually.
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Related errors
- 550 5.4.1 Recipient address rejected: Access denied · Microsoft 365 rejected an unknown recipient at the edge
- 550 5.7.708 Service unavailable. Access denied, traffic not accepted from this IP · Microsoft 365 blocked the tenant's outbound mail
- 530 5.7.57 Client not authenticated to send anonymous mail during MAIL FROM · The app sent to smtp.office365.com without logging in
- 535 5.7.139 Authentication unsuccessful, SmtpClientAuthentication is disabled for the Tenant · Microsoft 365 refused the SMTP AUTH login
- 550 5.7.520 Access denied, Your organization does not allow external forwarding · Automatic forwarding to an outside address is blocked
- 550 5.1.8 Access denied, bad outbound sender · The sending account was blocked for spam
- 552 5.2.2 The recipient's inbox is out of storage space · The recipient's mailbox is full
- 552 5.3.4 Your message exceeded Google's message size limits · The message is larger than the server accepts
- 535 5.7.8 Username and Password not accepted · The SMTP login was rejected
- 450 4.2.0 <user@example.com>: Recipient address rejected: Greylisted · The message was greylisted; it will be accepted on retry
- 553 5.7.1 <user@example.com>: Sender address rejected: not owned by user · You logged in as one user but sent as another address
- 530 5.7.0 Must issue a STARTTLS command first · The server requires TLS before it accepts mail