A big factor in the success of promotional emails, in my experience, is timing. By that, I am not referring to time for the day or when in the year (although those do matter in some situations), but rather the time in the user’s journey.
For example, I see promotional emails asking users to buy when the offer of a free trial might be more appropriate for where those users are in their journey.
To address this problem, you need to map the user journey and then identify timing or triggers that will allow you to identify when you should be sending what emails.