Howdy might be the normal way to greet someone where the plugin developer lives but it certainly isn't normal across the rest of our very big planet!
I have changed the code inside jetpack/modules/subscriptions.php from "Howdy" to "Hi" but still the introduction email says Howdy!!!
I searched and found many post comments etc asking how this "Howdy" word could be change but there was no developer feedback whatsoever as far as I could see.
It might as well say "Heyup", "Hiya", "Alright Mate", "Howrya" "G'Day" ... I hear you say "What the hell does that mean?" ... "Precisely" I say in reply.
It looks like such a great plugin and I would have given this five stars if it wasn't for this silly wording.