How to setup subdomain in DNS Manager?

Hi, I am assuming you have already added your domain to the DNS manager.

1. To create a subdomain, you need to add an A/AAAA record to your domain and add the name of the subdomain without the .example.com.

2. This is optional but you might also want to add a CNAME record with the www.subdomain.example.com

You can just an MX record for mailing.
If you want to create a mail.example.com, you can do that by repeating steps 1 and 2.

You should also consider adding DKIM, dmarc, and SPF records as TXT.

If the line above confuses you, do not worry about it unless your host instructs you to do so.

Yes, I have added A record for a subdomain in DNS manager in domain provider, It's work perfect but right now my focus on the mail system. my mail sending function is still loading, not working, I'm using Gmail SMTP for testing purpose. this mail function works perfectly in my local machine but not work in the Linode server. I thought about something setting I need to in Linode manager.
can you please more explain what I need to do for this function?

In an effort to limit spam, Linode requires you to open a support ticket and ask them to open your mailing ports, you must do the following first:

1. Configure valid A records and reverse DNS for the Linodes you’d like to use for mailing. (I am assuming you have already done this).

Then you could open a Support ticket and provide them with some basic information (the information they’ll ask for is outlined in the “Running a Mail Server” guide.)

I hope this helps. I personally run 3 Linodes with mailing ports open, it is not hard. Sometimes they will waive the rDNS requirement for you. Just make sure you do not spam and describe your setup.

Thanks mate! spyderweb
it's really great to help, I appreciate that!