Gmail's spam filter does a great job of keeping unwanted emails out of your inbox, but sometimes legitimate emails can end up in your spam folder by mistake. If this happens, you can mark the email as not spam to ensure that it gets delivered to your inbox in the future. Here's how:

1. Go to your spam folder. You can find it by clicking on the "Spam" label on the left side of your Gmail screen.
2. Find the email that you want to mark as not spam.
3. Check the box next to the email to select it.
4. Click the "Not spam" button at the top of the page. This will move the email from the spam folder to your inbox.
5. If you want to prevent this from happening in the future, you can add the sender's email address to your list of contacts. This will tell Gmail to deliver emails from that sender to your inbox, even if they are flagged as spam by the filter. To do this, simply click on the sender's name or email address and then click the "Add to contacts" button.

By following these steps, you can quickly and easily mark emails as not spam and ensure that you don't miss out on important messages.