Email limits when using "Send through External Email Services"
|Knowledge Article Number||000239596|
The Summer '16 release introduced the ability to send email using Lightning Experience through a Gmail or Office 365 account. Emails are recorded in Salesforce and sent directly through your companies Gmail or Office 365 account. The emails sent show up in both Salesforce and in your mail clients sent items. This method of sending email can help increase deliverability of emails sent from Salesforce because they exit directly from your own Gmail or Office 365 dedicated servers.
These mails are passed to Office 365 or Gmail using direct API calls and not SMTP connections. Mails sent using this method count against an organizations daily SingleEmailMessage limit.