Some very basic troubleshooting steps:
1. Start with the FQDN that your server is using.
   What is a proper FQDN? Almost any name in the format <hostname.>{subdomain(s).}<domain.><tld>
      Where <hostname> starts with a letter and does not represent the IP and
      where <domain.><tld> is a credible domain under your control.

2. Resolve that FQDN to IP(s) via public DNS.

3. Resolve all IP(s) back to that same FQDN via public rDNS.

4. Use only those IP(s) for outbound emails from that server.

5. Ensure that all inbound SMTP connections on those IP(s) see that server/FQDN.