HINT 01

You’ll need to have accessed Jess’s mailbox to find the recovered messages.

Look for the time.

Look for abbreviations.

Focus on the words that are only one letter long.

Every letter is replaced with its alphabetic opposite. A=Z, B=Y, C=X etc. and vice versa. So “OK” becomes “LP”, “PM” (mentioned after the time) becomes “KN”, and the word “I” becomes “R”.