550 5.7.1: Unknown Sender, Relay Denied or Not Allowed to Send As

A 550 5.7.1 bounce that mentions the sender or relaying means your own email provider refused to send the email, because of who it says it is from or how your email app signed in. The email never left. It is usually fixed in your email app’s settings or your account’s list of addresses, not by the person you were writing to.

550 5.7.1 is a general “refused by policy” code, so the sentence after it is what matters. The common ones about the sender are:

550 5.7.1 Unknown sender

550 5.7.1 Relaying denied
550 5.7.1 Unable to relay

550 5.7.1 Client does not have permissions to send as this sender

The wording varies by provider. If the sentence after 550 5.7.1 mentions a blocklist (“blocked using”, “listed on”), or says the email looks like spam, it is a different problem: see Why Emails Bounce.

• “Unknown sender”: the address in the From line isn’t one your account is allowed to send from. Your provider only lets you send as addresses it knows belong to you: your mailbox and any aliases set up on the account. Since Plusnet’s move to Greenby in late September 2026, this is the error Plusnet users see most.
• “Relaying denied” or “Unable to relay”: the outgoing server didn’t recognise you as one of its users, so it refused to pass your email on. Usually your email app isn’t signing in to the outgoing server, or is using the wrong server.
• “Client does not have permissions to send as this sender”: from Microsoft 365. You signed in correctly, but the From address belongs to another mailbox or shared mailbox, and you haven’t been given permission to send as it.

1. Sending from an alias or old address that isn’t on the account. For example sending as info@yourbusiness.co.uk through a mailbox that is yourname@yourbusiness.co.uk, or as an old provider address after a move.
2. Your provider has tightened its rules. An address that worked for years starts failing because the provider now checks the From address. Plusnet’s move to Greenby is the current example; my Plusnet and Greenby guide covers it step by step.
3. Sending your own domain’s email through your internet provider’s server. Many internet providers no longer let you send as another domain through their server.
4. The email app isn’t signing in to the outgoing server. “My outgoing server requires authentication” is unticked, or the password was changed and only the incoming side was updated.
5. A shared mailbox or another person’s address in Microsoft 365 without “Send as” permission.

1. Read the bounce. Note which address it says you sent from, and which of the sentences above follows 550 5.7.1.
2. For “Unknown sender”: add the address as an alias or extra mailbox on your email account, using your provider’s account settings, then send from it again. If the address is on your own domain and your provider won’t allow it, the long-term fix is a proper mailbox for that domain; my guide to email aliases after a migration explains the options.
3. For “Relaying denied”: in your email app, open the account’s outgoing (SMTP) server settings. Check the server name your provider gives, port 587 or 465, encryption on, “requires authentication” ticked, and the full email address and current password as the sign-in.
4. For “not allowed to send as”: ask your Microsoft 365 administrator to give you “Send as” permission for that mailbox. It can take up to an hour to work.
5. Send a test to an address outside your organisation.

Send from the same address to a personal Gmail address. It should arrive without a bounce. In Gmail, under the three dots and “Show original”, the From address should be the one you intended, and SPF and DKIM should show PASS. If they don’t, the email may now arrive but land in spam; see my guide to SPF, DKIM and DMARC.

Adding an alias or fixing a password is a five-minute job. It gets harder when you send your own domain’s email through an internet provider that no longer allows it, or when several people and devices send as one address: then the right fix is a new sending route and the records to go with it.

• Why Emails Bounce (and What the Bounce Message Is Telling You).
• Plusnet email not sending after the move to Greenby?.
• Is your domain on an email blacklist?, if the 5.7.1 bounce mentions a blocklist.

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Send me the bounce, and I’ll tell you which address or setting is being refused and put it right.