Note : This post is for .NET Core version 1.1 or .NET Standard 1.6. At that time, the SmtpClient class had not been ported across. If you are able to use .NET Core 2.0, then you are able to now use SmtpClient. Read this updated post for more info!
You’ve upgraded your latest project to the very latest version of .net core, but you can’t seem to find the correct SMTPClient namespace? It used to live in System.Net.Mail, but it’s just gone *poof*. What gives?
System.Net.Mail Is Not Ported (yet)
If you are on a .net core version 1.1 or less (Or you are working on a .net platform that implements the .net standard 1.6 or less), you will not have access to System.Net.Mail, which by extension means you will not have access to the SmtpClient class, or anything to read POP3/IMAP messages etc. They were not ported across (yet). Bummer!
However, on the Microsoft Github there is a pullrequest here for the port of System.Net.Mail that looks like it made it into .net Standard 2.0. That would point to the next version of .net core having SmtpClient back in. The release date is looking like early 2017.
So In The Meantime?
In the meantime many people are using the MailKit. It’s a very powerful library with a very similar interface/api to the original .net System.Net.Mail. In most cases you should be able to plug it in without too much hassle.