Why can't I connect via POP3, IMAP, or SMTP to a domain routed through Cloudflare?

Question

Why can't I send mail to a domain proxied behind Cloudflare? My mail client also can't connect to the server.

Answer

Cloudflare does not proxy POP3, IMAP, or SMTP traffic. The allowed ports are outlined in Cloudflare's documentation:

Network Ports | Cloudflare Docs

To allow mail to function:

1. Ensure your "mail" subdomain is added to Cloudflare as an "A" type record, not a "CNAME" type record.
2. Ensure the "mail" subdomain does not use the Cloudflare proxy feature. The "Cloud" icon should be grey, not orange.
3. The MX record for the domain should be set to the mail subdomain, not the base domain.