System.Net.Mail alternative without message size increase - email

Question

We are using the System.Net.Mail to send email messages as text with attachments. The attachments are Excel and Powerpoint files. The content types are set to the MIME types before sending the email.
A test done with three emails proved that the Exchange server recorded a 26% increase in each of the cases.
Is there a way to stop this increase in message size?
If not, is there a another .NET or open source alternative for this?
Would SMTP Drop overcome this issue?
UPDATE:
var mimeMessage = new MimeMessage();
mimeMessage.From.Add(new MailboxAddress(string.Empty, fromEmailAddress));
mimeMessage.To.Add(new MailboxAddress(string.Empty, toEmailAddress));
mimeMessage.Cc.Add(new MailboxAddress(string.Empty, copyEmailAddress));
mimeMessage.Subject = subject;
var builder = new BodyBuilder { HtmlBody = bodyText };
MvcApplication.Logger.Info("SendEmail:attachmentFiles:Count=" + attachmentFiles.Count);
foreach (var attachmentFile in attachmentFiles)
{
var attachment = new MimePart()
{
ContentObject = new ContentObject(File.OpenRead(attachmentFile)),
ContentDisposition = new ContentDisposition(ContentDisposition.Attachment),
ContentTransferEncoding = ContentEncoding.Binary,
FileName = Path.GetFileName(attachmentFile)
};
builder.Attachments.Add(attachment);
}
MvcApplication.Logger.Info("SendEmail:attachmentFiles:Added Count=" + builder.Attachments.Count);
mimeMessage.Body = builder.ToMessageBody();
using (var client = new SmtpClient())
{
client.Connect("smtp.domain.com", 25, false);
client.Send(mimeMessage);
MvcApplication.Logger.Info(
client.Capabilities.HasFlag(SmtpCapabilities.BinaryMime)
? "SMTP Server supports BinaryMime"
: "SMTP Server does NOT support BinaryMime");
client.Disconnect(true);
}
The above code sends a HTML message successfully.
The SMTP Server Capabilities flag for BinaryMime returns true.
If the ContentTransferEncoding is Base64 it works(9 excel and powerpoint files attached). If I change it Binary then just one corrupt excel file is attached. What am I missing here?

Answer 1

If the SMTP server supports the BINARYMIME extension, you could use MailKit to send email and set the Content-Transfer-Encoding of the attachments to ContentEncoding.Binary.
I don't think that System.Net.Mail supports the BINARYMIMNE SMTP extension (at least it didn't look like it did when I reviewed the referencesource on GitHub), hence System.Net.Mail.SmtpClient will always base64 or quoted-printable encode attachments.
The BINARYMIME SMTP extension allows clients to send messages without the need to abse64 or quoted-printable encode them.