I cannot find emails older than one year.
There can be several reasons for this but none of it will be due to LeanMail.
I suspect that you have an auto-archive function that removes those emails. It could be that your IT department has automated that.
Right-click on your folder/Properties/AutoArchive: Check if there is any auto archiving.
Another possibility is that your server only shows you the previous year of email. See below that I show 2 years.
To adjust: File/Account Settings/Account Settings (again)/Change:
If you want to ensure that it is not LeanMail (and I can assure you that it isn’t), you can disable LeanMail to see if anything changes.
Finally, my suggestion would be to contact IT for help on this.