https://apisof.net/. Specifically: https://apisof.net/catalog/System.Net.Mail.SmtpClient
This says that SmtpClient is available in .NET Core 2.0, and also .NET Standard 2.0.
If you know the specific API, I think you can use https://apisof.net (currently looks like the cert if invalid, though): https://apisof.net/catalog/System.Net.Mail.SmtpClient
You can also check the .cs sources in the netstandard/ref folder: https://github.com/dotnet/standard/blob/cc9f646354fc68a13707a82323d4032b8dbfda52/netstandard/ref/System.cs#L6124
This is the single source of truth since it's what the netstandard.dll reference assembly is built from.
@khellang If you're interested to know more about the certificate issue: https://github.com/terrajobst/apisof.net/issues/3.
Seems like your question is answered so closing.
Most helpful comment
https://apisof.net/. Specifically: https://apisof.net/catalog/System.Net.Mail.SmtpClient
This says that SmtpClient is available in .NET Core 2.0, and also .NET Standard 2.0.