block by email address?

Hello -

I'm going to implement an internal webmail system for users of a
website (think myspace as an example of what I need to do - i.e. mail
won't flow externally) and would rather just use open source mail
servers for doing it rather then having to build a database driven
messaging system. The only thing I'm not sure I can do is allow users
to block a sender so if user A doesn't want to receive messages from
user B they can block that user. Are there settings available in any
of the open source IMAP or SMTP servers that allow for this?

Thanks
Mike