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

Email marketer from Email Hippo answers that some validation tools do not permit the use of '+', even though it is perfectly valid, which can impact subscriptions, sign-ups, and opt-ins.

Email marketer from Litmus explains that subaddressing (using the plus sign) helps identify the source of email subscriptions. If a specific subaddress receives spam, you know which service sold your email address, however, some websites might not allow you to sign up using a plus sign.

Email marketer from Reddit suggests that the issue can originate either from the sender's or the recipient's side. On the sender's side, the email service provider (ESP) might not recognize the plus sign in the email address or may not be configured to handle it correctly. On the recipient's side, the email client or server might be stripping off the part of the address after the plus sign.

Email marketer from Webmaster Forum says that the server side or ESP may be the cause of the issue, meaning they don't see the plus side address as a unique address and could have the software stripping out anything after the plus. Suggests contacting the ESP.

Email marketer from SendGrid answers that subusers are not the same as a + sign, but some of the same things can be achieved using it, such as filtering purposes and monitoring.

Email marketer from Email on Acid shares that plus addressing is useful for filtering and tracking. However, some systems may not recognize or properly handle addresses with plus signs. If unsubscribe isn't working, it's likely the system isn't parsing the address correctly.

Email marketer from Reddit says that while Gmail supports '+', some email marketing services do not fully support it and it depends on the email service you are using. The issue may be the email service not recognizing the + in the address.

Email marketer from Stack Overflow mentions that the best thing to do is not strip the plus alias when storing the email address or when using it to unsubscribe. The unsubscribe feature should treat the plus alias email as different from the non-plus email.

Expert from Email Geeks shares that Gap was recently fined $200,000 by the CRTC over failing to properly manage unsubscribes. - <https://crtc.gc.ca/eng/archive/2021/ut211206.htm>

Expert from Word to the Wise shares that often the problem lies in an ESP not correctly matching email addresses with a plus sign. They may not know to strip information after the plus sign, like Gmail does.

Expert from Email Geeks says pluses in URLs may need to be escaped (%2b), or they’ll be treated as whitespace. If the ESP is generating the List-Unsubscribe header, and they’re not accepting it when it’s used with a plus sign, they’re the first place to ask.

Expert from Email Geeks suggests the ESP may not be correctly matching the email address without the +. Depends on if the email address is encoded in the mailto: link or not. Also, they “know” dots in a gmail address should be stripped before they process an unsubscribe, but they don’t “know” things after the + should be as well.

Expert from Spam Resource suggests an ESP may not handle the 'plus' sign, so it's up to the company to ensure that it can handle those addresses. Suggests testing the unsubscribe to make sure it actually removes the user from the list.

Documentation from RFC3464 highlights that the local-part of an email address (the part before the @ symbol) is interpreted according to domain naming conventions and may permit other characters. The plus sign is not specifically disallowed but the server on the receiving end has to recognise this convention.

Documentation from IETF discusses email address syntax and indicates that the local-part (before the @) can contain various characters, including the plus sign. However, the interpretation of these characters is up to the mail server.

Documentation from Mailchimp explains the complexities with email addresses and custom domains in particular, and the importance of working with your ESP to diagnose the email issue and to ensure a positive user experience for all end-users.

Documentation from Google Support explains that Gmail ignores periods in addresses and plus signs along with any characters after them. This means mail sent to `email@gmail.com` is the same as `e.mail@gmail.com` or `email+spam@gmail.com`.

Documentation from Fastmail explains that they fully support plus addressing but this is not the case for all mail services, and it might be because the receiving mail service does not support it or their address validation is not correct.