Does the order of mailto and https in List-Unsub header matter?
Summary
What email marketers say8Marketer opinions
Email marketer from Gmass explains the importance of the List-Unsubscribe header for deliverability but doesn't touch on mailto vs https order.
Email marketer from Litmus highlights the value of List-Unsubscribe, but the article does not reference an order of preference of the options.
Email marketer from Stackoverflow explains that there's no enforced preference by user agents when multiple options are present (mailto, http). The user agent is free to choose which method to use.
Email marketer from MailerLite highlights the value of List-Unsubscribe, without referring to any order of preferences for the options in the header.
Email marketer from Postmark covers implementing List-Unsubscribe headers but does not specify the order of the values within the header.
Email marketer from EmailOctopus explains that the List-Unsubscribe header helps improve deliverability, but doesn't specify order preferences for mailto: and HTTPS methods.
Email marketer from Mailjet explains that the List-Unsubscribe header allows recipients to easily unsubscribe and that providing both mailto: and HTTPS options is a good practice, however they do not specify an order requirement.
Email marketer from SendGrid recommends including a List-Unsubscribe header, and suggests including both mailto: and https: versions, but doesn't specify an order for the entries.
What the experts say3Expert opinions
Expert from Email Geeks explains that there is no specification meaning to the order of List-Unsub header. As far as RFC8058 is concerned, you have to have an https one and that’s it. And everyone who supports it will use that out of preference over the mailto:, regardless of order.
Email marketer from Email Geeks shares that no order is required for the List-Unsub header. The preference of what to use is up to the MUA.
Expert from Word to the Wise answers the question with “List-unsubscribe: <https://example.com/unsubscribe?email=$EMAIL> This is all you need to include in your email. If you feel you must also include the mailto: version, do not put it first. Some badly written unsubscribe parsers have been known to send an unsubscribe request to both addresses, and there isn’t any reason to make email more difficult.”
What the documentation says3Technical articles
Documentation from ietf.org specifies that the List-Unsubscribe header field is intended to provide a simple, one-click unsubscribe mechanism for mailing lists. It does not specify any particular order for the multiple unsubscribe methods (e.g., mailto:, http:).
Documentation from SparkPost details the List-Unsubscribe header, including implementation. They do not specify order, but suggest including both mailto and https versions.
Documentation from ietf.org defines a standard for HTTPS-based list unsubscribe. It mandates the presence of an HTTPS URL but doesn't specify the order when combined with other methods like mailto:.