Sending Email to SpecifiedPickupDirectory with MailKit

Question

I was using SmtpClient till now with ASP.NET MVC 5. For testing email send functionality on local system, I was using client.DeliveryMethod = SmtpDeliveryMethod.SpecifiedPickupDirectory;

Now, I want to do the same things in ASP.NET Core which does not have SmtpClient class implemented till now. All search for this ended up on MailKit. I have used their send mail code which is working fine with gmail.

I do not want to send testing emails each time and there may be a lot of scenarios in my project where I need to send email. How can I use the local email sending functionality with MailKit. Any links or little source code will help. Thanks

Answer 1

I'm not sure on the finer details of how SmtpDeliveryMethod.SpecifiedPickupDirectory works and what it does exactly, but I suspect it might just save the message in a directory where the local Exchange server periodically checks for mail to send out.

Assuming that's the case, you could do something like this:

public static void SaveToPickupDirectory (MimeMessage message, string pickupDirectory)
{
    do {
        // Generate a random file name to save the message to.
        var path = Path.Combine (pickupDirectory, Guid.NewGuid ().ToString () + ".eml");
        Stream stream;

        try {
            // Attempt to create the new file.
            stream = File.Open (path, FileMode.CreateNew);
        } catch (IOException) {
            // If the file already exists, try again with a new Guid.
            if (File.Exists (path))
                continue;

            // Otherwise, fail immediately since it probably means that there is
            // no graceful way to recover from this error.
            throw;
        }

        try {
            using (stream) {
                // IIS pickup directories expect the message to be "byte-stuffed"
                // which means that lines beginning with "." need to be escaped
                // by adding an extra "." to the beginning of the line.
                //
                // Use an SmtpDataFilter "byte-stuff" the message as it is written
                // to the file stream. This is the same process that an SmtpClient
                // would use when sending the message in a `DATA` command.
                using (var filtered = new FilteredStream (stream)) {
                    filtered.Add (new SmtpDataFilter ());

                    // Make sure to write the message in DOS (<CR><LF>) format.
                    var options = FormatOptions.Default.Clone ();
                    options.NewLineFormat = NewLineFormat.Dos;

                    message.WriteTo (options, filtered);
                    filtered.Flush ();
                    return;
                }
            }
        } catch {
            // An exception here probably means that the disk is full.
            //
            // Delete the file that was created above so that incomplete files are not
            // left behind for IIS to send accidentally.
            File.Delete (path);
            throw;
        }
    } while (true);
}

The above code snippet uses Guid.NewGuid () as a way of generating a temporary filename, but you can use whatever method you want (e.g. you could also opt to use message.MessageId + ".eml").

Based on Microsoft's referencesource, when SpecifiedPickupDirectory is used, they actually also use Guid.NewGuid ().ToString () + ".eml", so that's probably the way to go.