Do all email clients return custom headers for email replies?

Question

If I create a custom header in my email when sending, will all email clients return a 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

For completeness, rfc-822 and the In-Reply-To: header should be mentioned In-Reply-To: - although this is optional (indeed, applications are not required to add it).

If the title is present, it should be a reply letter.

Answer 2

You cannot guarantee that any client will do, so I would prefer no; especially because whether it’s to save custom headers in the specification or not, applications are not designed to not comply with the specifications here and there.

Thus, there is no guarantee, it depends on the implementation of customers.