I'm running my Postfix email server. I have only 1 A dns record for it: mail.my_domain.com I'm able to connect to it, receive and send email through it using only this url. Namely, I don't have "smtp...." and "imap.my_domain.com".
Will there be a necessity, advantage for me to create CNAME "smtp.my_domain.com" and "imap.my_domain.com" pointing to "mail.my_domain.com"?
Unless you plan on growing to the point you need to split your services onto different servers, these are unnecessary. When you do grow to that point, just add new A record for smtp, or even smtp1 and smtp2 if your volume is that high.
Most clients will discover your server for both IMAP and SMTP with doing the initial configuration. Even if you grow to the point of having different SMTP server for incoming an outgoing mail, you may want to continue accepting mail from authenticated users on the submission port (587) the mail domain.
If your use base grows to the point you need to have multiple IMAP servers, consider adding and imap domain posibly with multple addresses for multiple IMAP proxy servers.
No. These subdomains are completely optional.
Some mail clients will probe for these subdomains while setting up a new account, but they will typically fall back to mail, and/or allow you to enter a server name manually.
They are not necessary. I would describe them as medium suggestion/recommendation.
Per service DNS A/CNAME records smtp.example.com (outgoing authenticated email via smtp) and imap.example.com (incoming mail via imap) ease eventual future reconfiguration.
Such records allow to change servers (in DNS) without any need to reconfigure email clients.
Quite a few bigger domains use separate server for smtp-out, smtp-mx/smtp-in, imap/pop3.
