Hello,
I understand, that your server is affected by Spam and it is getting blacklisted.
I have 7 Years of Expertise in Microsoft Exchange Servers ( 2003/ 2007 / 2010 / 2013/ 2016). I have been working with Mail flow projects. Also with Edge servers and handled Many Spam related critical issues.
I can make sure, that there is no Spam entry and your Public Ip addresses doesn't get blocked by any RBL's.
Thanks,
Promod.V