Link Email To The Existing Issue
Open an email. Click on the drop-down list 'Select an Issue' at the Redmine Pane, chose the issue that you need and press 'OK' button.
After that will be opened a Task Details window with selected by you issue.
The email will be attached to the issue and available on 'Attachments' tab.
When you will open the email the Redmine Pane will be open the linked with it issue.
If was checked 'Add ID to email subject' option will be added an ID of the issue into email subject.