1. Point your domain address to a nameserver. Managing name server is difficult so usually you stick to something like cloudflare to both protect against DDoS and do the name server job.

2. You need to set the PTR Record or rDNS. both terms are the same and used interchangeably. There are 3 notes here:

•  use FQDN. www.domain.com is a FQDN, domain.com is NOT.
• PTR record should only return one FQDN and not more than that. otherwise spam detection might flag it as spam server
• The FQDN you set should not go through cloudflare proxy. in other word the PTR record you specify should directly point back to the server IP address. Thats why it is advised to not use www.domain.com as PTR record and use something like mail.domain.com so in case of DDoS attack your website would not get compromise.

3. MX record connects a domain to a mail server and it is like a CNAME, so domain.com MX record would be mail.domain.com and the A record of mail.domain.com should point to the mail server ip address. Note that cloudflare does not allow all port to pass through and only allows http ports like 80 and 443 to pass, thus if you run a mail server you should disable cloadflare proxy on the specific sub domain.

4. A record redirect domains to server IPv4 , AAAA record is the same except its for IPv6, CNAME simply map one domain to the other

5. To send or forward email you need to take care of SPF, DKIM and DMARK

• SPF: Specify what IP addresses are allowed to send emails from your domain.
• DMARC: Mention how the recipient mail server should treat mails that fail the DKIM verification
• DKIM: is a public/private key signing mechanism to verify the email is coming from the authorized author