Where is System.Net.Mail?

Dear all,

Trying to use SmtpClient and MailMessage here, however, the following situation occurs:
1) when hitting CTRL + . on any of them, it suggests to use System.Net.Mail
2) when accepting the suggestion, System.Net.Mail gets highlighted saying that System.Net does not contain Mail.

On closer inspection, actually typing System.Net.... does not reveal Mail or any other things.
Inspecting the .NET reference from the PCL project, and going to System.Net, all the things are there.

Did anyone bump into this in a Forms PCL project until now?

Thank you!