open source help desk software

I need an open source help desk and shared inbox software and its customization to meet the requirements. Previous knowledge and experience of customization of such suggested software is must.
I need an open source help desk and shared inbox software and its customization to meet the requirements. Previous knowledge and experience of customization of such suggested software is a must.
Some features needed as below:
1. Create an entry (call it an order or a ticket) in the system with an email
2. Amends the exiting ticket via email using some reference number in the subject line
3. Multiple groups to use the same system and have a dashboard to track the assigned or unassigned tickets
4. Each group may have one email or multiple emails to create/update the tickets
5. User logins and access to ticket depends on the group memberships (one user can have more than one group membership)
6. Some mechanism to notify users if
- A new ticket is created
- Tickets are not updated for certain period based on SLA defined
- Identify the tickets which failed to meet the SLAs and also identify the user who is assigned to such tickets
7. Multiple domain emails
8. Available on desktop and as a mobile app
9. Available to external user and customers