Hi,
please note that the From and Subject headers in the EML file are wrong - they contain non-ASCII characters, but are not using proper header encoding as specified by the MIME standard.
This generally means that applications parsing these messages are free to handle them as they wish. It looks like MS Outlook is lucky enough to use UTF-8 in this case, but Rebex Mail's default is the charset configured as default in the OS, which seems to be "windows-1250" (Central and Eastern Europe) in your case, and that unfortunately does not match the header charset.
Therefore, the proper solution to the issue described is to fix the application that created the mail message (as represented by the EML file). The correct MIME-compliant form of the Subject header is supposed to look like these, for example:
Subject: =?utf-8?Q?W=C3=B3zki_wid=C5=82owe_?=
Subject: =?windows-1250?Q?W=F3zki_wid=B3owe_?=
Rebex Mail R6.1 or later has an option that makes it use UTF-8 for non-ASCII headers as well:
var message = new MailMessage();
message.DefaultCharset = System.Text.Encoding.UTF8;
message.Settings.UseDefaultCharsetForHeaders = true;
message.Load("message.eml");
In earlier versions, use this instead:
var message = new MailMessage();
message.DefaultCharset = System.Text.Encoding.UTF8;
message.Options |= (MimeOptions)0x400000; // force default charset for non-ASCII headers
message.Load("message.eml");
Please let us know whether this successfully works around the issue.
Another issue in the EML message is lack of angle brackets around the e-mail address in the From header, which are mandatory.