Cannot send emails to addresses with Scandinavian

问题:

Using SmtpClient, MailMessage and MailAddress classes, I cannot send to email addresses such as å[e-mail]. I get the error/exceptions as shown below:

An invalid character was found in the mail header: 'å'.

--------------------------- Error sending email --------------------------- System.Net.Mail.SmtpException: The client or server is only configured for E-mail addresses with ASCII local-parts: å[e-mail].

at System.Net.Mail.MailAddress.GetUser(Boolean allowUnicode)

at System.Net.Mail.MailAddress.GetAddress(Boolean allowUnicode)

at System.Net.Mail.MailAddress.Encode(Int32 charsConsumed, Boolean allowUnicode)

at System.Net.Mail.MailAddressCollection.Encode(Int32 charsConsumed, Boolean allowUnicode) at System.Net.Mail.Message.PrepareHeaders(Boolean sendEnvelope, Boolean allowUnicode) at System.Net.Mail.Message.Send(BaseWriter writer, Boolean sendEnvelope, Boolean allowUnicode) at System.Net.Mail.SmtpClient.Send(MailMessage message)

These characters in my subject/body are fine, but not in email addresses.

I've tried setting SmtpClient.DeliveryMethod = SmtpDeliveryFormat.International, or MailMessage.HeadersEncoding = Encoding.Unicode (or UTF8) and it doesn't seem to change anything. These are real email addresses of people we need to communicate with so it's a bit of a problem.

I have been digging through the .Net source code but not really gotten anywhere. I tracked down a ServerSupportsEai property, and Google tells me EAI stands for Email Address Internationalisation (https://tools.ietf.org/html/rfc5336) but it's fairly unclear if this is a limitation in my code, or if the specific server I'm talking to just doesn't support this... since I'm using a test server to avoid sending emails to unsuspecting Swedish people!

Can someone help me clear this up - does .Net support this, if so what if anything should my client code do to enable it?

回答1:

To be able to to send UTF-8 characters according to RFC6531 both client and server need to support it.

If you use the .NET implementation of SmtpClient you'll need to target framework version 4.5 because that implementation supports the SMTPUTF8 extension.

If you set the DeliveryFormat property you have done what is needed for your client to support UTF8 characters:

using (var smtp = new SmtpClient())
{
    smtp.DeliveryFormat = SmtpDeliveryFormat.International;

    var message = new MailMessage(
        "[e-mail]",
        "ëçïƒÖ@example.com",
        "UTF8",
        "Is UTF8 supported?");

    smtp.Send(message);
}

If we reverse-engineer the Send method we can easily follow the stack-trace from your question. You'll find this implementation:

if (!allowUnicode && !MimeBasePart.IsAscii(this.userName, true))
{
    throw new SmtpException(SR.GetString("SmtpNonAsciiUserNotSupported", new object[]
    {
        this.Address
    }));
}

That allowUnicode boolean is being supplied from the Send method:

if (this.DeliveryMethod == SmtpDeliveryMethod.Network)
{
    return this.ServerSupportsEai && this.DeliveryFormat == SmtpDeliveryFormat.International;
}

Now here comes the server in the picture. When does the private boolean ServerSupportsEai becomes true? It turns out that on sending the EHLO command the SmptConnection calls ParseExtensions:

if (string.Compare(text, 0, "SMTPUTF8", 0, 8, StringComparison.OrdinalIgnoreCase) == 0)
{
    ((SmtpPooledStream)this.pooledStream).serverSupportsEai = true;
}

If you want to know upfront if your mail server is supporting that extension you can simply connect with a telnet client (I use putty) to your smtp server and send the EHLO somename command and inspect the results.

Connecting to smtp.gmail.comon port 587 gives me this output:

220 smtp.gmail.com ESMTP hx10sm19521922wjb.25 - gsmtp
EHLO fubar
250-smtp.gmail.com at your service, [2001:FFF:8bef:1:34d6:a247:FFFF:4620]
250-SIZE 35882577
250-8BITMIME
250-STARTTLS
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-CHUNKING
250 SMTPUTF8

Notice the last line: it holds our required SMTPUTF8

tl;dr

To be able to use international characters in the emailaddres make sure to set the DeliveryFormatto SmtpDeliveryFormat.International and use an smtp server that supports the SMTPUTF8 extension and advertises it in its EHLO command.