Forwarding mail through Cloudflare.
WHAT IT DOES
Receives mail for your domain and forwards it to another address, such as a personal Gmail account.
It replaces your MX records entirely.
WHEN IT MAKES SENSE
A domain with no hosting mailboxes, where you simply want mail forwarded A parked domain where you want to catch mail
WHEN IT DOES NOT
Any domain using hosting mailboxes. Enabling it takes over mail delivery and your mailboxes stop receiving.
This has surprised people who enabled it without realising what it replaces.
THE LIMITATION
It forwards; it does not provide mailboxes or sending.
You cannot send as your domain through it, which means replies come from whatever address you forwarded to.
That undermines the credibility of having a domain address at all.
WHAT WE PROVIDE INSTEAD
Full mailboxes on your domain, included with hosting, with proper sending and authentication.
For a business, that is preferable.
IF YOUR MAIL STOPPED AFTER A CLOUDFLARE CHANGE
Check whether Email Routing was enabled. It is one of the causes.