I spend at leas 30% of my time traveling. Quite often I receive e-mails from travel companies while on the road. I am on the mailing list for many hotels and airlines. What I find interesting is that while staying at hotel brand X, I receive e-mail from the same hotel brand telling me about specials in other hotels the company owns.
The one missing ingredient in my opinion is a thank you. I think these e-mails should be personalized and thank me for staying in the hotel as the e-mail is being sent. Sure this is more work but as a customer in a Brand X hotel it seems common courtesy to thank me for using the hotel, every time I am reached out to.
The concept needs to be embodied by the airlines and Amtrak as well. I just think it is good customer service.