The spam filters of Gmail, Outlook, Yahoo, & other email providers are very good, but sometimes they also make mistakes and flagging important emails also as spam. Here at below, providing some information about how to prevent the important emails to land in spam folder.
For most email programs and services, you have to whitelist each email by adding it to your contacts list.
In Gmail, for example, open an email from someone you never want to be flagged as a spammer and hit the down arrow next to the Reply button, then select "Add [person] to Contacts list". You can also use filters, for more advanced spam controls. Go to Settings > Filters > Create a new filter and you can add the email address, contact's name, just the domain (e.g., @yourdomain.com), and other criteria. Check the box "Never send it to Spam".
In Outlook/Windows Mail, add the email address to your Safe Senders list (under Tools > Options > Preferences > Junk E-mail). Also check the option to "Automatically add people I e-mail to the Safe Senders List".
In Yahoo!, have to set up a filter to prevent emails from being flagged as spam: Go to Mail Options > Filters and add a new one. In the "From header:" rule, add the email address and the "Choose Folder" to be your "Inbox".