Why does list-unsubscribe header not work with a plus sign in Gmail?

The failure of the list-unsubscribe header to function correctly with plus signs in Gmail stems from inconsistencies in how different email systems and ESPs handle these characters. While Gmail ignores plus signs and any characters following them, many systems don't recognize or properly parse addresses with plus signs. This can be due to ESPs not correctly matching plus-addressed emails, URL encoding issues, or validation tool limitations. The core problem lies in the lack of universal support and standardized interpretation of plus signs in email addresses, as highlighted by both technical documentation and expert opinions. Ultimately, ensuring proper unsubscribe functionality requires careful consideration of ESP configurations, address storage methods, and testing procedures, with a strong emphasis on compliance and user experience. If not managed correctly, there could be legal and financial ramifications.

The list-unsubscribe header might not work with a plus sign in Gmail due to a variety of reasons related to how email service providers (ESPs) and other systems handle plus addressing. While Gmail recognizes and correctly routes emails with plus signs, some systems may not properly parse, store, or process these addresses. This can lead to unsubscribe requests failing because the system doesn't recognize the plus-addressed email as distinct, or strips off the characters after the plus sign. Furthermore, some email validation tools may not even permit the use of plus signs, impacting subscriptions and opt-ins. The issue could stem from the sender's or recipient's side, with the ESP, server configuration, or email client being the culprit.

The issue of the list-unsubscribe header not working with a plus sign in Gmail often stems from the ESP (Email Service Provider) not correctly handling or matching email addresses containing a plus sign. Gmail ignores everything after the '+' sign but ESPs might not 'know' to do the same, leading to a failure to recognize the address for unsubscribing. Moreover, plus signs in URLs might need escaping, and if the ESP generates the List-Unsubscribe header and doesn't accept it with a plus sign, that's the first area to investigate. As a cautionary tale, Gap was fined for improperly managing unsubscribes, underscoring the importance of ensuring the unsubscribe process functions correctly.

The documentation collectively indicates that while Gmail ignores plus signs and characters following them in email addresses, not all mail services or systems adhere to this convention. RFC3464 and IETF documentation acknowledge that the local-part of an email can contain plus signs, but the interpretation is up to the mail server. Therefore, the issue with the list-unsubscribe header not working with a plus sign often arises because other systems or ESPs may not recognize or handle plus signs in the same way as Gmail. Mailchimp highlights the complexity and importance of ESPs properly diagnosing and addressing email issues, while Fastmail notes that full support for plus addressing isn't universal.