Loading from the IIS "pickup" folder - BCC and sender

[RichardD2]

With the imminent demise of the IIS SMTP Server component, I am writing a simple console application to pick up emails stored in a "pickup" folder, and send them on to our SMTP server. That way, I don't need to reconfigure our applications, or have them wait on a network request to send the email.

Using MimeMessage.Load to load the generated eml file gets us 99% of the way there. But there is a problem: neither the Sender nor the Bcc addresses are populated.

Looking at a raw message file, it seems that these properties are written as custom headers:

X-Sender: sender@domain.com
X-Receiver: to1@domain.com
X-Receiver: to2@domain.com
X-Receiver: cc1@domain.com
X-Receiver: cc2@domain.com
X-Receiver: bcc@domain.com
MIME-Version: 1.0
From: from@domain.com
To: to1@domain.com, to2@domain.com
Cc: cc1@domain.com, cc2@domain.com
...

Passing that file to MimeMessage.Load returns a message with the Sender set to null, and an empty Bcc collection.

So far, the workaround I've come up with involves a fair amount of code:

if (message.Headers["X-Sender"] is { } sender)
{
    message.Headers.RemoveAll("X-Sender");
    message.Sender ??= MailboxAddress.Parse(sender);
}

HashSet<string> recipients = new(StringComparer.Ordinal);
foreach (var header in message.Headers)
{
    if (!string.Equals(header.Field, "X-Receiver", StringComparison.Ordinal)) continue;
    
    var address = MailboxAddress.Parse(header.Value);
    recipients.Add(address.Address);
}

message.Headers.RemoveAll("X-Receiver");

foreach (var address in message.To.OfType<MailboxAddress>())
{
    recipients.Remove(address.Address);
}

foreach (var address in message.Cc.OfType<MailboxAddress>())
{
    recipients.Remove(address.Address);
}

foreach (var address in message.Bcc.OfType<MailboxAddress>())
{
    recipients.Remove(address.Address);
}

foreach (string recipient in recipients)
{
    message.Bcc.Add(MailboxAddress.Parse(recipient));
}

Is there some magic built-in way to load these values into a MimeMessage without manually parsing the headers?

[jstedfast]

The easiest solution is to do this:

MailboxAddress sender = null;
List<MailboxAddress>recipients = new List<MailboxAddress> ();

foreach (var header in message.Headers) {
    if (header.Field.Equals ("X-Sender", StringComparison.OrdinalIgnoreCase)) {
        if (sender != null)
            throw new Exception ("Too many X-Sender headers!");

        sender = MailboxAddress.Parse (header.RawValue);
    } else if (header.Field.Equals ("X-Receiver", StringComparison.OrdinalIgnoreCase)) {
        recipients.Add (MailboxAddress.Parse (header.RawValue));
    }
}

if (sender == null)
    throw new Exception ("No X-Sender header!");

if (recipients.Count == 0)
    throw new Exception ("No X-Receiver headers!");

var options = FormatOptions.Default.Clone ();
options.HiddenHeaders.Add ("X-Sender");
options.HiddenHeaders.Add ("X-Receiver");

smtpClient.Send (options, message, sender, recipients);

I recommend using Header.RawValue instead of Value if you intend to parse it using any of the address parsing routines because, internally, they will just convert the string back into a byte[] before parsing anyway. This saves unnecessary decoding/re-encoding and will result in better performance overall.

I also recommend, in this case, to use the SmtpClient.Send (or SendAsync) method that takes a sender and list of recipients rather than modifying the message to rewrite the address headers and risk losing information.

Instead of removing the headers from the message, you can also use the FormatOptions.HiddenHeaders property to tell the SmtpClient to drop those headers when it writes the message out to the SMTP server (effectively removing them).

[RichardD2]

Is the HiddenHeaders.Add overload which accepts a string part of an unreleased version? I'm using v4.17.0, and HiddenHeaders is a HashSet<HeaderId>, so it looks like you can't add the extended headers to the list.

[jstedfast]

Ugh, I forgot about that.

I guess you'll have to call Headers.RemoveAll() after all.