Currently, emails are associated with a project through the mail handler configuration, with all mail associated with an account sent to the same project.

Some customers would prefer to have a single mail address for their whole organization, like support@atlassian.com, and to have that mail automatically sorted to the appropriate project.

Or using Project Key to create issue in the email respectively, this will make more easier than configuring multiple mail handler and mail server.

One use case would be to have a mapping of users who are only allowed to use a single project, and to always create issues based on mail from that email addres in the associated project.