Go to Emails → Confirmation.
You can set:
Subject line — merge tags:
{{orgName}},{{campaignTitle}},{{amount}},{{donorFirstName}}Accent color — matches your brand in the receipt layout
Preview the message on that page, then send a test to yourself. Confirmations go out after a gift completes, using your SMTP from address.
If tests arrive but live donors do not, check spam, the from domain, and Emails aren’t sending.