Hello
AladdinJ! There are multiple steps you can take to improve email deliverability. We've published the following article that offers further guidance on getting your server's outbound email into the intended recipient's inbox:
Improving Email Deliverability
Regarding the HELO_DYNAMIC_SPLIT_IP, however, this may occur if accounts are configured to send mail via their specific IP rather than the server IP. Do you have accounts currently configured to send email via their own IPs? We look forward to hearing back from you!