Wrapper class for PHPMailer
As @ChoiZ already said in a comment, I would not pass the PHPMailer object in the constructor.

It doesn't really have any benefits (it's not like you can - or would want to - just pass a different mail class), and it's additional work for the caller. 

It also means that it will be hard to change the library you are using. With your Mail class, you created a nice interface for sending emails, but by passing the class that actually sends mails via constructor, you are binding yourself to that library.

Misc


The names body and plainText represent the same thing - just one with and one without HTML. It's generally best to name same things the same. So I would go with body/bodyPlain or htmlText/plainText to avoid confusion.
I would rename sendMail to mail, as the class is already named mail.
Your spacing is not consistent.
if there is not reason not to, fields should be private.