How to make a word bold etc in strings to be viewed in emails?

I know I can use @"stringtext etc" if I want to include strange characters such as html code. 

But I want to send an email from my app and I can only include the message as a string. I want to be able to make the message look nicer by making the headlines bold for example.

To explain the method call: first parameter is justa list of the users that recieves the mail,. The second is the email message and the third is just the  email subject.

 await _emailService.SendManyEmailsAsync(users, $"{profile.Firstname} {profile.Lastname} has made a new discussion. {model.Title} {model.Message}. http://linktodiscussion.com", "New discussion!");

This works but the email looks plain. The very least I want to do is make the title bold and make a new line after the title.  Any tips on how can I do this?