If you run a mail server under a dynamic ip, or an ip that should not be disclosed and you're proxiying with cloudflare, let's configure a 1€/month IONOS vps as mail relay server.
You will need a vanity MX subdomain mail.yourdomain.com
with SSL certificate. Each domain should use it as MX server, fix the SPF record and be advertised on SRV records if used.
The vps's IP should resolve to this subdomain. Ensure with the vps provider that the IP rDNS record points to your vanity subdomain, and in case of IONOS ask support to enable port 25, and allow on the external firewall.