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.