Hello, coders!
I wish to share with u the bug i've found.

Please correct me if I wrong, but it seems since unknown_user feature was added, there are possible troubles with spam email letters may occur.
Desire such installation:
We have system-wide email for responding "myredmine@mycompany.com";
We have several projects, "A", "B", and "C";
With our redmine we parsing different email addresses for adding issues to projects with uncnown_user=create and specific project= options enabled, including redmine email address, for example, projectb@mycompany.com, projectc@mycompany.com, myredmine@mycompany.com;

Once upon a day, we receive a spam letter where the sender equals recipient and unfortunately equals one of our emails..
For example, letter from projectc@mycompany.com to projectc@mycompany.com .
Then, application does the following:

creating user named projectc@mycompany.com according to spam letter;
sending letter 1 (with projectc's password) from myredmine@mycompany.com to projectc@mycompany.com;

creating issue 1 (with spam body) in project C;
sending letter 2++ (about issue 1 created) from myredmine@mycompany.com to projectc@mycompany.com,
which in cycle becomes new followups of issue 1;

creating issue 2 (with projectc's credentials) in project C;
sending letter 3++ (about issue 2 created) from myredmine@mycompany.com to projectc@mycompany.com;
which in cycle becomes new followups of issue 2;

creating user named myredmine@mycompany.com according to Letter 1;
sending letter 4 (with myredmine's password) from myredmine@mycompany.com to myredmine@mycompany.com;

creating issue 3 (with myredmine's credentials) in project A;
sending letter 5++ (about issue 3 created) from myredmine@mycompany.com to myredmine@mycompany.com;
which in cycle becomes new followups of issue 3...

Maybe this sequence is wrong, but algorythm is clear and from one hell letter (or because of lack of brains) we have 3 infinitely growing issues, which wasting rails time and eating disk space.

Only one solution I've found quickly - to add users with system and projects emails and uncheck all notification options for them.

Maybe the community offers another solution to prevent such email cycling.
Any suggestions?