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Editorial Team
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Asked: 2026-06-01T15:57:09+00:00

Can anyone recreate this issue? It seems to me like a quite serious bug

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Can anyone recreate this issue? It seems to me like a quite serious bug in SmtpClient (.NET 4.0) but I can’t believe no one has seen this before and Google doesn’t seem to show anyone seeing a similar issue.

When sending an email with more than 1 attachment and the ‘Attachment.Name’ property is used, the attachments will have the incorrect names (e.g. 2 attachments will have their names swapped). The work around (and actually probably the correct property to set) is to use ContentDisposition.FileName. But I would be very interested if this happens for everyone. Can anyone recreate this issue? It seems to me like a quite serious bug in SmtpClient (.NET 4.0) but I can’t believe no one has seen this before and Google doesn’t seem to show anyone seeing a similar issue. You’ll need to create a couple of zip files in c:\tmp\emailin\

var zipCt = new ContentType { MediaType = MediaTypeNames.Application.Zip };

var attachmentA = new Attachment(@"c:\tmp\emailin\a.zip", zipCt);
attachmentA.ContentDisposition.FileName = "a.zip";
attachmentA.Name = "a.zip";

var attachmentB = new Attachment(@"c:\tmp\emailin\b.zip", zipCt);
attachmentB.ContentDisposition.FileName = "b.zip";
attachmentB.Name = "b.zip";

var msg = new MailMessage("testfrom@example.com", "testto@example.com")
{
       Body = "body",
       Subject = "subject"
};
msg.Attachments.Add(attachmentA);
msg.Attachments.Add(attachmentB);

using (var smtp = new SmtpClient())
{
     smtp.DeliveryMethod = SmtpDeliveryMethod.SpecifiedPickupDirectory;
     smtp.PickupDirectoryLocation = @"c:\tmp\emailout\";
     smtp.Send(msg);
}

If you now look at the eml file in c:\tmp\emailout\ you will see something like

X-Sender: testfrom@example.com
X-Receiver: testto@example.com
MIME-Version: 1.0
From: testfrom@example.com
To: testto@example.com
Date: 11 Apr 2012 12:36:48 +0100
Subject: subject
Content-Type: multipart/mixed; boundary=--boundary_0_1b7bb1ee-ba28-4258-b662-554adb7ff81a


----boundary_0_1b7bb1ee-ba28-4258-b662-554adb7ff81a
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: quoted-printable

body
----boundary_0_1b7bb1ee-ba28-4258-b662-554adb7ff81a
Content-Type: application/zip; name=b.zip
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=a.zip

UEsDBAoAAAAAAG5ki0AAAAAAAAAAAAAAAAAFAAAAYS50eHRQSwECPwAKAAAAAABu
ZItAAAAAAAAAAAAAAAAABQAkAAAAAAAAACAAAAAAAAAAYS50eHQKACAAAAAAAAEA
GADa2JQw1xfNAdrYlDDXF80B2tiUMNcXzQFQSwUGAAAAAAEAAQBXAAAAIwAAAAAA
----boundary_0_1b7bb1ee-ba28-4258-b662-554adb7ff81a
Content-Type: application/zip; name=a.zip
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=b.zip

UEsDBAoAAAAAAHZki0AAAAAAAAAAAAAAAAAFAAAAYi50eHRQSwECPwAKAAAAAAB2
ZItAAAAAAAAAAAAAAAAABQAkAAAAAAAAACAAAAAAAAAAYi50eHQKACAAAAAAAAEA
GAD67/k51xfNAfrv+TnXF80B2tiUMNcXzQFQSwUGAAAAAAEAAQBXAAAAIwAAAAAA
----boundary_0_1b7bb1ee-ba28-4258-b662-554adb7ff81a--

Note how the Content-Type: and Content-Disposition: file names do not match for each attachment.

Am I doing something wrong? Is this a bug that I should log with MS?

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1 Answer

  1. Editorial Team

    This is because you require a new instance of ContentType for each attachment.

    var zipCt = new ContentType { MediaType = MediaTypeNames.Application.Zip };
var zipCt2 = new ContentType { MediaType = MediaTypeNames.Application.Zip };

var attachmentA = new Attachment(@"c:\tmp\emailin\a.zip", zipCt);
attachmentA.ContentDisposition.FileName = "a.zip";
attachmentA.Name = "a.zip";

var attachmentB = new Attachment(@"c:\tmp\emailin\b.zip", zipCt2);
attachmentB.ContentDisposition.FileName = "b.zip";
attachmentB.Name = "b.zip";

    Should fix your issue.

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