Gmail automatically identifies spam and other suspicious emails and sends them to Spam.  Sometimes they incorrectly flag legitimate emails.  In those cases, you should unmark those messages as spam.  When you report an email as not spam, this will help teach Gmail to not mark emails like that as spam in the future.

Unmark an email as spam

1. On your computer, open Gmail.
2. On the left, click More.
3. Click Spam.
4. Open the email.
5. At the top, click Not spam.

Tip: Beyond reporting an email as not spam, to stop a message from being sent to Spam in the future, you can:

• Add the sender to your Contacts.
• Filter these messages.

For more information on the spam warning labels, see Gmail's Mark or unmark Spam in Gmail support article.