Environment:
  Redmine version                3.1.0.stable
  Ruby version                   2.0.0-p645 (2015-04-13) [i386-mingw32]
  Rails version                  4.2.3
  Environment                    production
  Database adapter               SQLServer

I turned on "Allow issue assignment to groups" option in settings.
My project has 3 members: user1, user2 and group1.

1. I created an issue from email, not assigned:

   rake redmine:email:receive_pop3 unknown_user=accept host=comp.local port=110 username=redmine@comp.local password=XXX allow_override=project,tracker,priority project=test

2. I received email notification from Redmine about that issue:

   Issue #1198 has been reported by Ryabinovskiy Alexander.

Work #1198: test issue from email
• Author: Ryabinovskiy Alexander
• Status: New
• Priority: Normal
• Assignee:
• Category:
• Target version:
Test issue description

1. I replied to that email with quoting, my comment was added to issue, but "Assignee" was changed to group1 !

   Updated by Ryabinovskiy Alexander less than a minute ago

Assignee set to group1
Comment Edit
Test comment 1

Issue #1198 has been reported by Ryabinovskiy Alexander.

Work #1198: test issue from email
Author: Ryabinovskiy Alexander

Category:

Test issue description

Also, Status, Priority, Target version was parsed from email, but I used only allow_override=project,tracker,priority option!