Do email clients all return custom headers on emai

Question

If I create a custom header in my email when sending, will all email clients return the custom header when someone replies to the email?

Ex)

MailMessage mail = new MailMessage();
mail.To = "[e-mail]";
mail.From = "[e-mail]";
mail.Subject = "this is a test email.";
mail.Body = "this is my test email body.";
mail.Headers.Add( "X-Company", "My Company" );  <---- This is my custom header.
SmtpMail.SmtpServer = "localhost";  //your real server goes here
SmtpMail.Send( mail );

Thanks

Answer 1

You cannot guarantee what any given client will do, so I'd hazard a guess at "no"; particularly so since, whether retaining of custom headers is in the specification or not, applications are wont for not adhering to specifications here and there.

So, there is no guarantee, it is up to the clients implementation.

Answer 2

For completeness, rfc-822 and In-Reply-To: header should be mentioned - though it's optional (indeed applications are not obliged to add it).

If the header is present, it must be a replied email.