host apc.olc.protection.outlook.com [52.101.9.13] SMTP error from remote mail server after pipelined end of data: 550 5.7.1 Unfortunately, messages from [198.252.99.13] weren't sent.

Question

host apc.olc.protection.outlook.com [52.101.9.13]

    SMTP error from remote mail server after pipelined end of data:

    550 5.7.1 Unfortunately, messages from [198.252.99.13] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150). You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors. [Name=Protocol Filter Agent][AGT=PFA][MxId=11BCD75FBB87DBFF] [BL02EPF0001A101.namprd05.prod.outlook.com 2026-02-17T08:48:03.635Z 08DE6BAFE00D0232]

I have contacted my ISP and they say this is an Outlook issue? any one know fix?

Answer 1

The error message you're encountering, 550 5.7.1 Unfortunately, messages from [198.252.99.13] weren't sent, indicates that the IP address from which you are trying to send emails is on a block list used by Outlook. This typically happens when the sending IP address has been flagged for suspicious activity or has a poor reputation.

To resolve this issue, you can take the following steps:

1. Contact your Internet Service Provider (ISP): Since the error message suggests that part of their network is on the block list, your ISP may need to investigate and possibly request removal from the block list.
2. Use the delist portal: If your ISP confirms that the issue is related to the block list, they can use the delist portal to request removal from the blocked senders list. They can forward the NDR message to delist@microsoft.com for assistance.
3. Check IP reputation: Ensure that your sending IP address has a good reputation and is not being used for spam or malicious activities. You can check the reputation of your IP address through various online tools.

If these steps do not resolve the issue, you may need to follow up with Microsoft support for further assistance.

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References:

• Fix NDR error "550 5.7.1" in Exchange Online
• Use the delist portal to remove yourself from the blocked senders list