ASP.NET - How to fix - The type or namespace name 'MailMessage' could not be found (are you missing a using directive or an assembly reference?)

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The type or namespace name 'MailMessage' could not be found (are you missing a using directive or an assembly reference?)   

Replace

MailMessage message = new MailMessage();

with

System.Net.Mail.MailMessage message = new System.Net.Mail.MailMessage();