Closed Mail Domain

Hi

1-What is this domain for?

mail.mydomain.com

I have access to mail server via this address: https://<my IP>:8090/rainloop/

2-I know its came from the Create Mail Domain from crating website , but if I not choosing the cheek box , am I able to use rainloop email in my server?

you should edit entire “system” (if im not wrong)
e.g.
edit the zone
@ IN MX domain.tld

i have tried it at 2 different vps
cyberpanel 2.1.1 (but different installation date and update)
both need (only) modification at dns zone

and only that. now i use ticked create mail domain

ref: We (still) need to enable auto create mail subdomain or not? - Support and Discussion / General Discussion - CyberPanel Community

create the subdomain uder the main domain and you can use that must be the subdomain of main domain