While checking a variety of sources it seems to me there are a couple possible causes.
There are reports of email over 10 years old being deleted.
There are responses from Microsoft reps in their forum that if you don't login to an account often enough, such as every week or month, older emails might start to get deleted.
Microsoft doesn't have a pay option to keep the oldest emails. My Gmail appears to have my oldest emails.
Maybe
getting your own domain name and using a service such as
LuxSci would meet your requirements.