Gmail Emails Going to Spam: How to Fix It

Try these in order

1. Mark it Not spam. Open Gmail → More → Spam, tick the email, and click Not spam at the top. Future emails from that sender should go to your inbox (Gmail Help).
2. Add the sender to your contacts. In Google Contacts, click Create contact and save their address — Gmail won't send their mail to Spam (Gmail Help).
3. Create a 'Never send to Spam' filter. Click Show search options, enter the sender's address next to From, click Create filter, tick Never send it to Spam, and save (Gmail Help).
4. Check you haven't blocked them. Blocked senders go straight to Spam. Unblock them under Settings → Filters and Blocked Addresses (Gmail Help).
5. Ask the sender to check their setup. If legitimate mail still goes to Spam, the sender may not be following Google's sender guidelines (Google Help).