My task is to create mechanism which change email subject by adding extra note at the beginning.
when I want to send something to group called "All" with subject "Hi" it should be changed to "[All] Hi".

Do you have any idea how can I do it?
It would also be great to control who can send mail to which group (same can only receive mail, but not write) but it's less important.

I started developing this using Events but it looks like there is no possibility (I haven't found) to use this for groups - it's just for users. And also when somebody send an email to the group then I don't know that it is from there - I just have an original sender.