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550 5.7.513: blocked by recipient’s customer block list

550 5.7.513Client host blocked using Customer Block listPermanent: retrying will not helpFrom Microsoft 365 and Outlook.com

Typical wording
550 5.7.513 Service unavailable, Client host [203.0.113.40] blocked by example.com using Customer Block list

What it means

The wording of 5.7.513 is unusually direct: it names your connecting IP, the recipient domain, and the phrase "Customer Block list". That list is the recipient tenant’s own IP block list in Exchange Online Protection, kept by their admins.

So the decision was made inside one organization. Microsoft does not maintain it and the delist portal cannot touch it. It is closely related to 5.7.507, but 5.7.513 states outright that a customer configured block caused the refusal.

Only the recipient’s Microsoft 365 administrator can remove the entry. As the sender you can ask, explain, and fix whatever led to the block.

Likely causes

  • The recipient’s admin added your IP to the IP Block list in the connection filter policy.
  • Your mail shares an IP with a sender the recipient wanted to block, typical on shared hosting.
  • An old block was never cleaned up after a past incident or a reassigned IP.

How to fix it

  1. Note the IP and domain named in the bounce and confirm the IP is your current outbound address.
  2. Ask the recipient by phone or another address to remove the IP from the connection filter policy in the Microsoft Defender portal.
  3. If the IP is shared, move to a dedicated IP or a relay so the recipient can trust your traffic separately.
  4. Once they confirm the change, send a single test message and check that it arrives.

Checks that apply

  • Blacklist Check If your IP is on public lists too, the recipient may have copied that judgement; cleaning them up helps your case.
  • Whois Lookup Shows who owns your domain and IP context so you can explain to the recipient who you are.
  • MX Lookup Confirms the recipient is on Exchange Online, where this customer block list lives.
See the example diagnosed

The classification runs without network lookups and we never store your paste.

FAQ

Will the Microsoft delist portal help with 5.7.513?
No. The block belongs to the recipient tenant, so only its admins can remove it.
Can I avoid it by changing my From address?
No. The list matches your connecting IP, so the address in the message does not matter.

Microsoft 365 delist portal Outlook.com SNDS

All 191 bounce codes