JETI executes a transition on the issue that is associated with the email that is being processed.
Use Case #1:
JETI mail handler is processing an email that is associated with an Issue that is Closed. JETI besides adding a comment to the issue, may execute a "Reopen" transition to reopen the issue for further investigation.
Use Case #2:
JETI is processing an email and creates a new issue in status Open, after the issue is created it executes a transition to move the Issue to new status called "Raised via email".
The transition to execute is identified by a transition property called "jeti.on.emai.transition". JETI executes the first transition that has this property in the status the issue is in.
Also you must enable "Execute transiton" attribute in the Mail Handler. See attached screenshot.