Setup the SendGrid Driver with FluentSMTP

In SendGrid

1. Sign in at app.sendgrid.com.
2. Verify a Sender Identity (domain authentication is better than a single sender).
3. Settings → API Keys → Create API Key. Name it FluentSMTP. Permission: Restricted Access with Mail Send enabled is enough. Full Access also works, and is more than you need.
4. Copy the key once.

In FluentSMTP

Settings → FluentSMTP, pick SendGrid.

• From Email must be allowed by that SendGrid account (authenticated domain or verified sender).
• Paste the API Key.

Leave Store in Database selected. You can tick Disable Encryption, or switch to Store in wp-config.php. Both are optional. See Where secrets are stored.

Save Connection Settings, send a test.

The form links to Create API Key.