Kindly, may I ask if your A mail record (DNS only) at Cloudflare dashboard?
Do you have an MX record for yourdomain.com which points to mail.yourdomain.com?
Is your e-mail service allowed to send/receive e-mails?
Which ports do you use at your host/origin?
Have you got any TXT record for SPF, DKIM and/or DMARC setup at Cloudflare dashboard?
Do you use an SSL connection or?
How about, have you tried following the steps here: