How to set DNS to work with VPS email server

Question

I have a VPS in one provider (Provider A), and I have set the email server all works ok. in another provider (Provider B) I have the domain registered. So, let's assume it is mydomain.com

On the provider B: I had created a subdomain called mail.mydomain.com, configured the dns as follow:

HOST         Type    Result
(empty)      A       123.123.123.123
www          A       123.123.123.123
mail         A       123.123.123.123

Then on mail exchange records I've configured as follow:

HOST            Priority   Result
(empty)         30         123.123.123.123

On the subdomain mail.mydomain.com, I have configured the dns as follow:

HOST         Type    Result
(empty)      A       123.123.123.123
www          A       123.123.123.123

Answer 1

1. Set up your mail server(s) to listen at mail.mydomain.com
2. Make sure your firewall doesn't block your mail server(s)'s ports
3. Set an A record on mail.mydomain.com pointing to your IP address
4. Set a MX record on mydomain.com pointing to mail.mydomain.com
5. It should be working, but you probably wanna set up DKIM, DMARC and SPF