I've requested the approval of my account, and got the following response back:

"The email sending domain must have an active and functional website so that we can proceed with the review."

The website I have provided has been up and running almost non stop for the past 3 years, and the domain has been ours for at least that long as well. I'm not sure why the team over there couldn't access the website, it's not blocked in any country as far as we know, and anyone can access it freely.

Update: Solved, thank you for reviewing the information again.