ok, then go to your domain’s DNS settings (where the DNS is hosted), and create A record of mail.yourdomain.tld to your Server IP (without any proxy if using cloudflare to manage DNS).
Create MX record and point to mail.yourdomain.tld. Additionally you can create SPF TXT record, DKIM record, DMARC policy TXT record to make email settings perfect.
This should solve your issue.