If the datacenter was forced to tell you about it themselves, then your IP has its reputation already destroyed and blacklisted many places. You need to fix it fast before the damage is hard to revert. Some email providers simply completely block sending your mails to them. Happened to me once with Outlook. They rejected all my emails and had to go through a lengthy process to whitelist me again after I corrected my server's configuration. Good thing that the IP was only blacklisted with them, no place else.
First of all, check your /var/log/maillog
Paste it here:
https://pastebin.com/Change Paste Exposure to "Unlisted", Create new paste and post here the link.
Notice: Everything is in the log. IP addresses, email addresses, and maybe other sensitive data. If it's ok for you, then share the link here. If not, then just PM it to me if you like