Okay, I just created a PR to add new setting max_allowed_message_recipients with default value of 30. Some highlights:
- A (non-staff) user can send a message to maximum 30 recipients by default.
- If user adds a group as message recipient and if that group has more than 30 members, then this setting will come into action and an error will be generated.
- This setting will not come into action if a message was sent to a group via email.