Uniquely identify an email on send using System.Net.Mail

Question

I am looking for a way to uniquely identify an email at the time it is sent using the .NET SmtpClient class (in System.Net.Mail), ie

        SmtpClient client = new SmtpClient(smtpServer, smtpPort);

        client.EnableSsl = false;

        MailAddress from = new MailAddress(fromAddress);
        MailAddress to = new MailAddress(destinationAddress); 

        MailMessage message = new MailMessage(from, to);

        message.Body = body;
        message.Subject = subject;

etc.

I can set the 'Message-Id' property using message.Headers (set it to a System.Guid) but it gets overwritten by the smtp server, and I dont know how to access the message id that the smtp server generates.

I need a client system to open the pop3 mailbox that receives the email and be able to track it down by this unique identifier.

Anyone know if this is possible?

Answer 1

One option is to attach a custom header to the message. Custom headers are indicated with a X- prefix. So in this case, you might create one called X-Unique-Id which would not be overwritten.

However, it's not necessarily the case that all mail clients will allow filtering/searching based on custom headers, you'd have to check the client.

Per MSDN:

Any custom headers added will be included when the MailMessage instance is sent.

Just add to the message.Headers collection like:

message.Headers["X-Unique-Id"] = "1";

Another option would be to use a special reply-to: or from: value that includes the message id. E.g. if your reply address was [email protected], then change it to auto+{unique-id}@responder.net. Many mail servers (including gmail) will treat the + as a wildcard and deliver the message to [email protected] as normal. This is a common approach used for allowing users to reply directly to in-app messages via email.