What Does a 550 5.7.1 Bounce Mean, and How Do I Fix It?

What it means

550 is a permanent failure and 5.7.1 is a generic "not allowed" status. The code alone tells you little: several unrelated problems share it. The text that follows the code is what matters, so copy the full bounce before you change anything.

Because it is permanent, don't keep retrying the same message to the same address. Fix the cause first.

Gmail variants

Google lists these 550 5.7.1 texts in its SMTP error reference, among others:

Google's sender guidelines add one more: a 550 5.7.1 rejection "because the sending server's IP address is on an IP suspended list. You might get this error if you're sending email using a shared IP with a poor reputation."

Microsoft 365 variants

Microsoft says 5.7.1 "indicates a security setting in your organization or the recipient's organization is preventing your message from reaching the recipient." For cold senders the variants that matter are:

• "Client host [xxx.xxx.xxx.xxx] blocked using Blocklist 1": your sending IP is on Microsoft's blocklist. Forward the bounce to delist@microsoft.com or use the delist portal.

• "Delivery not authorized" or a group that accepts only members: a recipient-side restriction. Microsoft notes that "Typically, you can't fix the problem yourself." Remove the address from the campaign.

• "Unable to relay" or "Client was not authenticated": a configuration problem on the sending or receiving server, usually an MX or relay setting.

Microsoft also uses its own codes for newer rules, such as 5.7.515 for failed authentication. See Outlook blocking cold email.

Yahoo

Yahoo describes its permanent errors as 553 and 554. Its error code page lists invalid addresses, failed DMARC or DKIM, content policy and a Spamhaus listing of your IP among the causes, and says not to retry an email that comes back with a 5xx error.

What to do

1. Group your bounces by exact text, not only by code.

2. If most say reputation or unsolicited mail, check whether a neutral email from the same mailboxes is also refused or sent to spam. If yes, the IPs or the domain are the problem, not the copy.

3. If the text names headers, PTR or authentication, fix the setup. These are not reputation problems.

4. Remove addresses that return recipient-side restrictions.

5. Watch the bounce rate per mailbox for a few days after the fix. See bounce rate too high.

What we recommend at Outreach2day

We set up SPF, DKIM and DMARC on every domain automatically. That removes the authentication causes, not the reputation ones, which is why we recommend up to 25 cold emails per mailbox a day and warm-up for every mailbox.