Jul 06, 2019 01:46 PM|mgebhard|LINK
This new issue has nothing to do with ASP.NET. My best guess is you have a rule set in the Outlook client or you are using POP3 which moves the email from the server to the client.
Secondly, there is nothing wrong with using SmtpClient considering you are using Web Forms which is a legacy framework. SmtpClient totally appropriate for Web Forms and the reason I recommend SmtpClient.