So you've had an email come into your spam on Gmail, and you want to make sure that it doesn't happen again.
There are two things you should do.
The quickest and easiest step you can take is to report the email as not spam. This flags to Gmail that they made a mistake with their spam filter.
This will likely work on its own.
Open the email, and click the "Not Spam" button
Add a filter to stop emails from this email address going into spam.
Open the email, click the drop down in the top right of the email, and click "Filter messages like these".
A box will pop open with the email address already filled out. Click the "Create Filter" button at the bottom.
Select "Never send it to Spam" and click "Create Filter".
Are you looking for help on a different email provider to Google?
Here are some other providers we have guides for: