This was a tough call. Tumblr has a lot of separate infrastructure from Yahoo (including a completely separate password reset system that sends a link from no-reply@tumblr.com), but ultimately it's using Yahoo's account system (the registration page at https://www.tumblr.com/register just links to a slightly-stripped-down version of Yahoo's main registration form), so I decided to defer to Yahoo for everything (except the login page / form).
This was a tough call. Tumblr has a lot of separate infrastructure from Yahoo (including a completely separate password reset system that sends a link from no-reply@tumblr.com), but ultimately it's using Yahoo's account system (the registration page at https://www.tumblr.com/register just links to a slightly-stripped-down version of Yahoo's main registration form), so I decided to defer to Yahoo for everything (except the login page / form).