ASP.NET Identity Confirmation Email Is Plain Text Instead of HTML

The problem more than likely isn't with your Identity, it is probably just with the mail function itself. Have you set the MailMessage.IsBodyHtml property to true?

MailMessage message = new MailMessage(fromEmail, toEmail, subject, body);
message.IsBodyHtml = true; // here

This will set the body of the email to be rendered as HTML. By default, the property is false, so you need to explicitly declare it to be true.

The issue is likely with the mail service/api you're using. I have used several and they generally have either an optional htmlBody parameter or an isBodyHtml parameter. If you specify which one you're using, or provide the code for the SendAsync Methos in the EmailService class (in IdentityConfig.cs), it'll probably be easy to point you in the right direction.

If your stuck with plain text, you can send plain text with a url and the client will often convert it into a link for the user. Outlook and gmail do this.

So, it would look like this instead:

await UserManager.SendEmailAsync(user.Id, "Confirm your account", "Please confirm your account:" + callbackUrl);