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Editorial Team
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Asked: 2026-05-27T20:11:35+00:00

Using the MailMessage class in .NET 4, I found an issue today that I’m

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Using the MailMessage class in .NET 4, I found an issue today that I’m unable to resolve so far. Please see the following code:

using (var message = new MailMessage())
{
    message.From = new MailAddress(@"uwe.keim@gmail.com", "Uwe Keim");
    message.Bcc.Add(new MailAddress(@"uk@zeta-software.de", "Uwe Keim"));

    // This fails (see screenshot).
    /*1*/ message.To.Add(new MailAddress(@"uk2@zeta-sw.net", "Müller, Fred"));

    // This succeeds.
    /*2*/ message.To.Add(new MailAddress(@"uk2@zeta-sw.net", "Fred Müller"));

    // This also succeeds.
    /*3*/ message.To.Add(new MailAddress(@"uk2@zeta-sw.net", "Muller, Fred"));

    message.Subject = "Test";
    message.Body = "Some text body.";

    new SmtpClient().Send(message);
}

This is a simple snippet so send an SMTP message. Mutually trying the lines /*1*/, /*2*/ and /*3*/, the behavior differs:

Whenever a receiver (“To”) name contains a German umlaut (i.e. “Ä”, “Ö” or “Ü”) and a comma (i.e. “,”), the receiver sees damaged text in the email message he receives:

enter image description here

As you can see in the above screenshot (taken from Outlook 2010), there is a cryptic “=?utf-8?Q?M=C3=BCller” in the “To:” line.

Leaving out the comma or removing the German umlaut fixes this. I’ve tried both Exchange 2003 and hmailserver to get the same result.

My question is:

Is anyone aware of this behaviour and has a solution to it?

Update 1:

As suggested by user Adam Maras, I fired up Microsoft Network Monitor while sending the email message.

To me, it seems that the MailMessage class (or the SmtpClient class?) is already doing it wrong:

enter image description here

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1 Answer

  1. Editorial Team

    So, after some digging, I came across Microsoft Support article 2576045: FIX: Email client does not support a name in a delivery address field that is encoded by the MailMessage class in the .NET Framework 4 if the name contains a non-ASCII character.

    It appears that, when writing out an address that contains Unicode characters, the MailMessage class does not correctly encode something. I certainly can’t tell you what it is based on the KB article’s information, but whatever it is, it’s enough to make downstream readers choke on the headers.

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