@Fred,

Mail.dll does not remove custom headers. All email headers are available using IMail.Headers collection. Are you sure this header is there (examine the text of this attachment)? Please contact us directly if you believe this is a parser problem.

Additionally you can make your code a bit more readable:

MimeRfc822 rfc822 = (MimeRfc822) mail.Attachments[1];
IMail attached = rfc822.Message;

This code remove Non standard Header? :

byte[] eml = email.Attachments[1].Data;
IMail attachedMessage = new MailBuilder().CreateFromEml(eml);

@Math

Neither IMAP protocol nor Mail.dll has such limit.
As for the servers, some, like Gmail, limit certain operations, but it happens rather on search level and usually is configurable.

@Paul

You are correct. In most cases there is just a single instance of particular email header.
You can use GetValues method to access multiple headers:

List<string> values = mail.Headers.GetValues("Received");

@Paul,

Headers collection is not case sensitive.
This means that email.Document.Root.Headers[“Delivered-To”] and email.Document.Root.Headers[“delivered-to”] return same header value.