Javamail changing charset of subject line

Problem solved!

mail.setSubject(MimeUtility.encodeText(subject, "utf-8", "B"));

solves it and sends utf-8 encoded mail subjects.

Why there is that "B" why there isn't ISO-something?

You should use setSubject(String subject, String charset) which is a convenient function for this purpose.

Session session=Session.getDefaultInstance(new Properties());
MimeMessage mimeMsg= new MimeMessage(session);
String subject="Herr Müller reist nach \u0141\u00f3d\u017a.";
mimeMsg.setSubject(subject,"utf-8");
System.out.println(subject);
System.out.println(mimeMsg.getHeader("Subject")[0]);

In MimeUtility it is said:

There are a set of methods to encode and decode MIME headers as per RFC 2047. Note that, in general, these methods are not needed when using methods such as setSubject and setRecipients; JavaMail will automatically encode and decode data when using these "higher level" methods. The methods below are only needed when maniuplating raw MIME headers using setHeader and getHeader methods.

From my point of view, Message.setSubject should be the entry point for this purpose.

The cp1252 in your subject encoding shows up, because it is your standard encoding on your platform.

Your posted example is the "result" of

mail.setSubject(MimeUtility.encodeText(subject, "cp1252", "Q"));`

Solved.

mail.setSubject(MimeUtility.encodeText(subject, "utf-8", "B"));

solves it and sends utf-8 encoded mail subjects. \n/

the legal values for "encoding" are "Q" and "B"... The "Q" encoding is recommended for use when most of the characters to be encoded are in the ASCII character set; otherwise, the "B" encoding should be used.

See https://www.rfc-editor.org/rfc/rfc2047.