Strange increase in email size using SMTP class to send an email as opposed to outlook

I am sending an email with an attachment using the following Code

Dim msg As New System.Net.Mail.MailMessage(req.EmailFrom, req.EmailTo)
            Dim att As New System.Net.Mail.Attachment("C:\Documents and Settings\michaelr\Desktop\1216259.pdf")
            With msg
                .Attachments.Add(att)
                .Body = req.EmailBody
                .Subject = req.EmailSubject
            End With
    Dim client As New System.Net.Mail.SmtpClient()
            client.Host = PDFService(Of T).mSMTPServer
            client.Send(msg)

The file size of the attachment is 396KB, upon the recipient receiving the email outlook shows the file size as 543Kb. Strange thing is if I send an email with the same attachment using outlook the file size is 396Kb.

I understand that file sizes can increase due to the attachment being base 64 encoded as opposed to just raw binary.

What I am failing to see is why outlook send a file which is 396KB in size but in code when sending it, the same file is 543Kb.

Any help would be appreciated and get a big green tic开发者_JS百科k.

This increase looks like an overhead of MIME encoding to me

Here's a guess: Outlook may perform some kind of compression on the attachment, or may use a more efficient encoding mechanism than the built-in MailMessage class does.

UPDATE: Looks like Outlook uses a proprietary encoding mechanism: http://support.microsoft.com/kb/290809