SmtpClient发送邮件遭遇The specified string is not in the form required for a subject.

通过System.Net.Mail发送邮件，出现错误：

System.ArgumentException: The specified string is not in the form required for a subject.
   at System.Net.Mail.Message.set_Subject(String value)
   at System.Net.Mail.MailMessage.set_Subject(String value)
通过Reflector代码看到 System.Net.Mail.MailMessage 的Suject调用了内部方法MailBnfHelper.HasCROrLF 
internal string Subject
{
    get
    {
        return this.subject;
    }
    set
    {
        if ((value != null) && MailBnfHelper.HasCROrLF(value))
        {
            throw new ArgumentException(SR.GetString("MailSubjectInvalidFormat"));
        }
        this.subject = value;
        if (((this.subject != null) && (this.subjectEncoding == null)) && !MimeBasePart.IsAscii(this.subject, false))
        {
            this.subjectEncoding = Encoding.GetEncoding("utf-8");
        }
    }
}

相应的解决方案是采用下述代码

message.Subject = subject.Replace('\r', ' ').Replace('\n', ' ')

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