The issue of Gmail's List-Unsubscribe sometimes requiring a second attempt is multifaceted. It appears that Gmail might be phasing out full support for mailto: unsubscribe methods, prioritizing URL-based unsubscribes instead. Low sender reputation, engagement rates, and spam complaints trigger delays or hinder the initial unsubscribe attempt. Server-side misconfigurations, especially those affecting the handling of POST requests, are a common cause. Rendering issues, variations in email client handling, and potential security measures implemented by Gmail, such as two-click unsubscribe, also contribute to the problem. Inconsistent server responses and failure to adhere to RFC specifications for List-Unsubscribe implementation exacerbate the issue. There is also the suggestion that Gmail may be actively testing unsub functionality, which is influencing the responses.