First: get your account manager involved. They can and will go to bat for you to help you through things like this.

Second: I am highly suspicious that they gave you “no reason”. I’ve been denied in the past several times and each time a straightforward reason was given. More and more these days they are (rightly) requiring more attention be paid to how you will handle bounces, spam complaints, etc. if you don’t have a good plan for that, you’d likely get denied. Without more info, that’s as good of a guess as anyone other than you can make.