I can not send and receive any emails on my email server

[Lucius]

Hello. I built a website with VestaCP in my VPS, which enable the mail service function. The Roundcube webmail works fine. But when I setup on my other email client like Gmail or Blue Mail on my phone, I can not send or receive any emails. When trying to send, I encounter a 451 Temporary Error. When I edit my email account configuration on VestaCP, I saw both imap and smtp servers are just showing like example.tld instead of imap.example.tld/smtp.example.tld. I configured the DNS records like these:

example.tld A record points to my IP;
example.tld. MX record points to mail.example.tld;
imap.example.tld A record points to my IP;
pop3.example.tld A record points to my IP;
smtp.example.tld A record points to my IP;
mail.example.tld A record points to my IP.


Can anyone tell me is there anything wrong with my configuration? Because the 451 error is about wrong configuration about server so I am guessing something incorrect existing on my DNS configuration.

Thanks for any advice.