Regarding XML file for email service, I am not familiar with this one, but regarding e-mail functionallity when using Cloudflare, I suggest you to take a look on the article below:
The DNS record(s) used for e-mail service (usually the A mail record) should be cloud at DNS tab on Cloudflare dashboard for your domain and the MX record should point to a hostname that is set to that unproxied (usually the A mail ) with DNS only record, otherwise your e-mails will not work as it seems to be the case.
The A mail record should contain the value of an IP address of your hosting provider/email server.
the hostname (DNS record) used for e-mail send/receive should be cloud (usually it’s the A mail ), while the MX record should point to a hostname (usually the A mail ) that is set to cloud
Cloudflare’s default configuration only allows proxying of HTTP traffic and will break mail traffic.
Just in case, sharing here a list of some websites/tools I usually use to check, test or even generate something in case I forgot how does it go regarding an e-mail setup: