OK here is what you need to do
0. Most important. Fix the problem. It is no good trying to get de-listed if their own logs tell them you are still emitting spam. You may need to be able to tell people what happened and what you have done to fix it.
1. It is no good asking them why you are blocked, unless you have definitively determined that it is a specific decision taken by them in your case (even automatically). More likely, you are blocked because they use a reputation service.
2. So you need to check your status on ALL commonly used reputation services.
3. What is a reputation service? It is someone's opinion, based on their published policies, that your IP address or email domain meets the criteria to be listed on that service - usually the criteria can be determined automatically, but sometimes the lists are curated manually. In other words, it is an expression of opinion, not an instruction to anyone to block you. Mail operators may choose to use such lists to block outright, or as part of a scoring system, and usually in combination with a whitelist/blacklist of their own. (For example you would usually whitelist your bigger customers - you don't want to lose an order for a million widgets just because an over-enthusiastic salesperson got your customer listed on one of these lists).
4. So find out who has listed you.
Check both your IP address and email addresses against all blacklists. Robtex is a service which can do this for you:
So for example if your email domain is theregister.co.uk and your mail server is aspmx.l.google.com, then bung the IP address 22.214.171.124 into the box at the top, then hit the "blacklists" link and it will tell you if you are listed by any blacklists.
Do the same with the domain.
5. Then you need to jump through the relevant hoops with each and every blacklist which has listed you. In most cases you can get de-listed (once) by asking. But not in all cases. Some will only de-list you after a month - but these are little-used.
Generally, if you can get off all the lists, you will find you can get mail delivered again. But that's the first thing you need to do.
An alternative would be to actually move your mail domain to Google, or Outlook.com. They already do all the rate-limiting, outbound filtering of spam and other defence-in-depth measures you will need, and have developed relationships with all the other large mail providers to report abusive users.