A) Each email should have a subject.
So in my opinion not sending without a subject is correct behaviour.
At least it is impolite to write none, as the receiver has to open the email and read it in order to see if it is anything she wants to read.

B) What shall it display, when there is no subject?
"None", "Empty", "No subject given"?

C) Yes this really is a bug. it should either reply with
Re: Untitled
or with
Re:

A) I disagree. It may be impolite to send a message without a subject but it is not the job of a mail client to force the inclusion of a subject. As far as I can tell from limited testing, the SOGo web mail client is unique in doing this. Thunderbird, Mac Mail, GMail, pine, mailx, squirrelmail all allow messages without a subject. Some of these clients ask "Are you sure you want an empty Subject?" but some don't. Surely the internet is about the freedom to do stuff like send emails without a subject :-)

B) It should display nothing - like Thunderbird for example. It should definitely not alter the content of the subject header because this is misleading. If the content is null, that's what should be shown.

C) I agree. I think it should be "Re:" (without the quotes)

We disagree on A), so let us see what others think, especially Invers.

B) Now I understand you. Yes, it could/should be empty, if you allow empty subjects.

We agree on C).

RFC 2822 Section 3.6.5 has Subject as an optional "Information field":

http://tools.ietf.org/html/rfc2822

Good argument.

Fixed in revision 28cde64403e65a1ebeda3a984650fd31d8614f29 and f51ae70ff345179926dfdd4fd5856e968bdfc327.