When journals falsely assume I already have my PhD, do I need to correct them?
Include your proper title and position in your email signature.
I'd say not to explicitly correct them in the email, but to include a signature to the email that says something like &quot;Mr/Ms Sursula, PhD Candidate at Faculty of ABC, XYZ University&quot;. Ideally  you'd want to use your university's official email footer format to do so.
That way, you can inform them of how you're properly meant to be addressed, without openly chastising them for getting it wrong.