This is a message for all the startups which feel superior to huge companies. Yes, it's a real struggle to keep email design patterns consistent across different departments or even sub businesses. Usually it requires a huge effort just to find out what are the email types you're sending out. And actually changing them to look alike is only step #2 or #3.
You can easily find examples for every corporation out there. Let's take a look at Apple which is definitely an iconic company in terms of design and keeping things updated. Each email type they've sent me looks different, only one one is optimized for smartphones.