You can control whether or not drop-in email receipts are sent to students and Providers from the Admin Panel's Settings.
The default for this setting is Student Drop-In email receipts: Enabled, Provider Drop-In email receipts: Disabled, and Share Provider feedback in Drop-In email receipts: Disabled. These are the same default settings as with session receipts.
More information on this feature is available in the Help Center article, Drop-In Settings.