This article only applies if you use Gmail.
Gmail has full control over what emails they decide to put into your spam folder. Gmail may, on occasions, put perfectly legitimate emails into your spam folder, such as website contact form emails, or comment / moderation emails. Therefore, it’s important you check your spam folder regularly.
However, you can setup a rule within Gmail, which tells Gmail to always move certain messages into your inbox (e.g. if the subject line matches certain criteria, then you can specify “Never send it to Spam”):
Please note, if you need any further support on using Gmail, you should seek support from Gmail directly. You can visit their support area at https://support.google.com/mail.