Why am I getting a 5.7.1 delivery not authorized error in Gmail due to duplicate headers?

Email marketer from Reddit shares that using a proper email sending library or service usually prevents duplicate header issues because they handle header formatting automatically. The user also highlights that manual construction of email headers is prone to errors.

Marketer from Email Geeks explains that the error indicates duplicate headers, which are not permitted by RFC5322, and offers to help further if the headers are posted.

Email marketer from SendGrid explains that a 5.7.1 means that email has been rejected by a server, to solve this you must correct any duplicated header fields which goes against conventions.

Email marketer from Email on Acid explains that Error 5.7.1 usually indicates a general delivery issue. To resolve this, it is important to avoid spam filters by not duplicating fields, and following best sending practices.

Marketer from Email Geeks shares that the issue is most likely caused by special characters in the friendly from.

Marketer from Email Geeks shares that Gmail has started rejecting messages with duplicate 'To :' or 'Date :' headers as an attempt to curb DKIM relay spam.

Email marketer from Google explains that the 5.7.1 error often indicates a policy violation or unusual sending patterns. While not directly addressing duplicate headers, it highlights the importance of adhering to Gmail's sending guidelines to avoid delivery problems.

Email marketer from Litmus explains that some older email clients could be confused when reading duplicated headers which can cause email sending servers to reject it as it vioaltes conventions.

Email marketer from Mailjet explains the error code 5.7.1 means the e-mail could not be delivered because of authorization or security issues. To resolve this, avoid spam-like content that duplicate header fields, and review your email sending practices.

Email marketer from Stack Overflow explains that the 550 5.7.1 error generally signifies a problem with the recipient's server configuration or policies and could be due to unusual message formatting.

Expert from Spam Resource explains that Gmail is enforcing RFC 5322 more strictly, causing bounces due to issues like duplicate headers. The bounces indicate 'Our system has detected that this message is not RFC 5322 compliant' and suggest reviewing the specifications.

Expert from Word to the Wise reports that getting a Gmail 5.7.1 code is an indicator to look at SMTP errors. This means you must correct invalid, malformed or duplicated SMTP header fields.

Expert from Email Geeks mentions that a duplicate Message ID header can also cause this issue.

Documentation from Microsoft says that to ensure messages are correctly routed and delivered, it is important to avoid headers such as 'To', 'From', 'Subject', and 'Date' being duplicated. Servers reject messages because this violates standards.

Documentation from RFC Editor specifies that certain headers like 'To', 'From', 'Subject', and 'Date' should appear only once in a message. Duplicate headers violate this specification, leading to potential rejection by email servers.

Documentation from SocketLabs details that a 5.7.1 SMTP error specifically means 'Delivery not authorized, message refused'. This indicates the recipient server has policies preventing delivery (which can be triggered by malformed headers).