If you are writing any server script such as php or asp to send email from your server / hosting to people's mail box, the email will go into their junk / spam box, how to make the email go to inbox instead of junk / spam box? To do that, just follow the steps below :
1) Login to your server / hosting cpanel.
2) Click MX Entry (see the picture below for reference) :

3) Chose your domain from the drop down list.
4) Wait for finish loading the page.
5) Type 0 on the field of Priority, then type mail.yourdomain.com onto the field of Destination. (for example, if your domain is flish.com, you can type mail.flish.com onto the field of Destination). Then click "Add New Record" button.

6) Go back to the homepage of your cpanel, then click Email Authentication (see the picture below for reference) :

7) Enable it then wait it finish loading the page.

8) On the field of Additional Hosts that send mail for your domains (A), click Add button, then type mail.yourdomain.com

9) On the field of Additional MX servers for your domains (MX), click Add button, then type mail.yourdomain.com again

10) On the field of Additional Ip blocks for your domains (IP4), your server or hosting is already auto assign the ip address. (You can go to http://mxtoolbox.com to check whether or not the ip address of your mail.yourdomain.com is correct or not.

11) Scroll down to the page and click Update button.