Thank you for contacting Sitefinity support.
Unfortunately I am not able to provide you with accurate data on email campaigns performance. As you mentioned, the speed of sending emails depends at most on the SMTP specifications.
The way Sitefinity works is as follows: it schedules an asynchronous task and starts sending the messages to the server. It updates the stats only in case there is an SMTP response on the SendMail method indicating success and that means that the email is successfully delivered to the server. The system then goes by all the queued items in the database and runs the NotificationTask to send mail. We process against different types of SMTP messages as well and any SMTP exception outside of a successful delivery is not marked as delivered.
I hope this information has been useful to you and would be glad to provide you with further help. Thank you in advance.
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