the question is about sending (not receiving!) mails.
For the mail receiving without discover your real IP you can use separate mail server pointed by MX record - this is very easy.
Now, assumed we have a forum board. The hosting server real IP is protected with Cloudflare DNS.
But, if the forum board sends mails to its users, the real IP can be easy discovered by analyzing mail headers.
This way hackers can discover your real IP and DDOS you directly to the IP of the hosting server bypassing Cloudflare.
The question is how to send mails from the forum board without discover your real IP?
If the board uses standard php mail command, the real IP will be packed into the header by hosting server itself.
If the forum board uses a separate server with SMTP account (i.e. Zoho, etc), the server puts the real IP into the header as well.
I suppose I need a kind of “anonymized” SMTP server.
Do you have any idea how to send mails without discover your real IP?