5.7.1 Delivery not authorized, message refused

Gmail

• 550 5.7.1The user or domain that you are sending to (or from) has a policy that prohibits the email that you sent. Contact your domain administrator for assistance.

  A rule set by the administrator of the recipient's (or the sender's) domain prohibits the message. Only the administrator can change it.

• 550 5.7.1Invalid credentials for relay ip-address. The IP address you've registered in your Workspace SMTP Relay service doesn't match the domain of the account this email is being sent from. If you are trying to relay email from a domain that isn't registered under your Workspace account or has empty envelope-from:, you must configure your email server either to use SMTP AUTH to identify the sending domain or to present one of your domain names in the HELO or EHLO command.

  Google Workspace's SMTP relay did not recognise the sender: the IP address or the HELO and MAIL FROM domains do not match the relay's configuration.

• 550 5.7.1This message is likely unsolicited email. To reduce the amount of spam sent to Gmail, this message has been blocked.

  Gmail considered the message likely unsolicited and blocked it. Check with Mail delivery test.

• 550 5.7.1This message does not meet IPv6 sending guidelines regarding PTR records and authentication.

  The message sent over IPv6 does not meet Gmail's IPv6 requirements on PTR records and authentication. Check with Mail delivery test.

• 550 5.7.1This message is likely suspicious due to the very low reputation of the sending IP address. To best protect our users from spam, the message has been blocked.

  The sending IP address has a very low reputation at Gmail, so its mail is limited or blocked. Check blocklists, authentication and what you send; Postmaster Tools shows the reputation. Check with Blacklist check.

• 550 5.7.1This message is likely suspicious due to the very low reputation of the sending domain. To best protect our users from spam, the message has been blocked.

  The sending domain has a very low reputation at Gmail, and the message was blocked. Check with Blacklist check.

• 550 5.7.1Messages missing a valid address in the From: header, or having no From: header, are not accepted.

  The message has no From field, or the address in it is not valid. Check with Header analyzer.

• 550 5.7.1Messages missing a valid Message-ID: header are not accepted.

  The message has no valid Message-ID field, which Gmail requires. Check with Header analyzer.

• 550 5.7.1Messages with multiple addresses in the From: header are not accepted.

  The From field holds several addresses, which Gmail does not accept. Check with Header analyzer.

• 550 5.7.1The message contains a unicode character in a disallowed header.

  A header field holds Unicode characters where Gmail does not allow them. Check with Header analyzer.

• 550 5.7.1This message is not RFC 5322 compliant because it has duplicate headers. To reduce the amount of spam sent to Gmail, this message has been blocked.

  The message repeats header fields that must appear only once, so it does not follow RFC 5322. Check with Header analyzer.

• 550 5.7.1This message is not RFC 5322 compliant because the From: header is missing. To reduce the amount of spam sent to Gmail, this message has been blocked.

  The message has no From field, or the address in it is not valid. Check with Header analyzer.

• 550 5.7.1This message is not RFC 5322 compliant because it has multiple From: headers. To reduce the amount of spam sent to Gmail, this message has been blocked.

  The message has more than one From field; RFC 5322 allows only one. Check with Header analyzer.

• 550 5.7.1This message is not RFC 5322 compliant because the From: header has a non-compliant domain name. To reduce the amount of spam sent to Gmail, this message has been blocked.

  The domain in the From address is not a valid domain name. Check with Header analyzer.

• 550 5.7.1This email has been rate limited.

  The message was refused because of Gmail's sending and receiving limits.

• 550 5.7.1The IP you're using to send email is not authorized to send email directly to our servers. Use the SMTP relay at your service provider instead.

  The IP address is not allowed to send directly to Gmail's servers; send through your provider's SMTP server.

• 550 5.7.1Daily SMTP relay sending limit exceeded for this customer.

  The daily limit of Google Workspace's SMTP relay service was reached for this user or for the organisation.

• 550 5.7.1Encoded-word syntax is not permitted in message header header-name. To reduce the amount of spam sent to Gmail, this message has been blocked.

  A header field uses encoded-word syntax (=?…?=) where Gmail does not allow it. Check with Header analyzer.

• 550 5.7.1This message is not RFC 5322 compliant. There are multiple header-name headers. To reduce the amount of spam sent to Gmail, this message has been blocked.

  The message repeats header fields that must appear only once, so it does not follow RFC 5322. Check with Header analyzer.

• 550 5.7.1This message is not RFC 5322 compliant. There is a malformed header- name header. To reduce the amount of spam sent to Gmail, this message has been blocked.

  A header field is malformed. Check with Header analyzer.

Microsoft 365 / Exchange Online

• 5.7.1Delivery not authorized

  The sender is not allowed to send to this recipient.

• 5.7.1Unable to relay

  The server is not the message's final destination and refuses to relay it.

• 5.7.1Client was not authenticated

  The server requires authentication before sending, and the client did not authenticate.