Congratulations on taking action to get free of a toxic environment!
A lot of people just put up with it.
This is advice for next time (for you) and for any others that might be considering this:
Set your out of office and your voice mail greeting just before you turn in your notice.
Don't say anything nasty towards the company, just a simple:
"I am no longer available.
You can contact [whoever] to get a new representative."
Replace [whoever] with your boss's name, email of the sales department, or anything generic (not a specific person unless that person is a manager/supervisor in charge of your work).
You'll note I said "just before" because if they walk you out, you may not have a chance to do it just after you turn in your notice.
While this isn't a direct answer to your question, I do think it is helpful enough to not just be a comment
(and it is my comment on another answer)
Bringing the following comment in to my answer:
Since any company will already have access to all own email accounts regardless of the particular situation the OP is in, this is pointless, since any company computer admin can legally reverse the change
No isn't pointless - you do this as part of a professional exit.
You are pointing the customer to the next person, so they get the best customer service.
You are not "pulling one over on the company in case they pretend you still work there" because, really... how many companies would do this? (I'm expecting it is a very small number)
If you are in a very high touch situation and know it would be traumatic for the customer to be handed over more than once - then maybe you shouldn't set your out of office and redo your voice mail prompt.
It isn't about screwing your current company, it is about helping your customers, and it is about YOU and YOUR professionalism (whether your company deserves it is not relevant)