Another open question is regarding a help system. Is it planned for RC? Should there be an additional object "RCHelp" to which I can append plugin depended help messages? Is providing help for plugins mandatory...
I have no idea, but what exactly are you thinking about? Error messages, or do you think one needs to add pointers in the interface, like "click here to send an email" and so on?
A general help contains information for end user. Basic informations about what a mail system is, how to fill out "To" field, FAQ, which options were available for what purpose and such things (and indeed on which button to click to get a mail send).
Another good idea you pointed out is help for error message. Message window is not very large. In some languages, you need more words to provide same information. So a extended error message help would also help administrator as well as users to understand what had happened (something like "IMAP does not respond" is hard to understand if you've limited knowledge about how mail systems works).
Button help can be provided as ALT-text. This should be a MUST for all plugin developer. It makes no sense to provide such a facility in core code and new buttons (provided by plugins) have no text explanation (this would also help RC to increase accessibility for people with disabilities).
Plugin help is more specific. I created a plugin for SquirrelMail which enables end-user to specify expiration days for mailboxes. I got a couple of demands to provide end user help for the plugin (e.g. What is mail expiration? Are the mails really deleted? Is recovery available?).
I know, it is sometimes boring for coder to provide sufficient help text, but I would appreciate such a function for the RC project. If this approach can eliminate a couple of end-user-questions from mailing list, it would be beneficial for all.
(One of my favorites on mailing lists were questions about "My mails disappeared, can you please recover?!" :-))
I know this is not easy to implement and maintain; I don't know how much work we need to get that up and running, but I think you should at minimum keep these things in mind, as you work on concept and code.
Live long and prosper Florian http://www.qv90.de