It's a business soliciting letter. We get those all the time
from unknown companies looking for customers.
If the letter is not addressed to a specific person, name. It's
not worth agonising over it.
Just open it up, and toss it away into the recycle bin if you
are not interested in their offerings.
If you are in a HDB you are the lessee, the HDB is the lessor.
You are not owner of the HDB property, you are only owner of the
LEASE.
Okay then... haha... my mum's dropping it in the "returned mail"
haha.