Some providers use the reputation of the mail servers (smtp.domain.com) and not the actually domain (your-domain.com).
Static IPv4 address (make sure it's not blacklisted)
Not behind a CGNAT
Ability to change the PTR record of your IP (usually though the support of your ISP)
Ability to open common mailserver Ports (25,143,465,587,993)
Because above is only needed if you want to communicate with people using Gmail, Yahoo, Outlook, Fastmail, Proton etc (The monopoly).