A common setup is to have one common "INBOX project" (usually 1st line support in the company) where you collect all support emails and then transfer them over to other projects in your account.
To do this follow the steps below
- You should first create an "INBOX project" and setup the ticket system to monitor your support mailbox (such as firstname.lastname@example.org). All support messages will land in this project.
- For every other project you should configure the ticket system exactly like the "INBOX project" as in 1.) above, but you should not check the checkbox 'Enable monitoring of the e-mail account'
Now, when a new support message is sent to your mailbox a new issue will be created in the 'INBOX project'. You can then transfer the issue to any other project and then perform 'Reply to correspondent' to send answers to your customer as usual.
If you get an incoming email reply on an issue in a project that is not the 'INBOX project', as long as the ticket-Id is present, the ticketing system will know that it should attach the reply to the existing issue in the other project. So, once an issue is transfered, you can continue the email conversation in that issue.