emails deleted

I have been a cox customer for 15 years or more. I recently went to find some old invoices on my email and found that all emails over 2 months old had been deleted. I am only up to 12% capacity. What happened? Is this because I check emails on my phone and rarely on my computer? Some type of warning would have been nice rather than cox deleting all my emails for me. Or at least deleting a few starting with the oldest ones not just throwing everything out!  I really need those old invoices- is there any way of retrieving them? Do I have to just go to using Gmail exclusively?