“It’s a good idea to do this with transactional emails, like receipts, to make it very easy for users to find and reference them. But the tactic can be useful for newsletters and marketing emails too if you send them on a daily, weekly or monthly basis. Remember, you want your recipients to look forward to your emails so much that it doesn’t matter what the subject says.”
This an unconventional approach, but it’s one that can be quite effective. Your readers will learn to recognize and trust you. Of course, you will have to deliver content within those emails that is different and valuable (but the same in terms of what can be expected from you).