I run a mail server with no spam filtering and its not too bad, maybe 2-3 spam messages a day and Thunderbird catches those. I just have a somewhat strict postfix configuration with graylisting enabled.
My email is only ~13 years old so I'm not sure how bad it is for people with older domains/emails.
Spam can be hard to predict. I know people who have had a setup like that (working just fine as for you), and then all of a sudden they've been overwhelmed with hundreds to thousands of spam mails per day.
My email is only ~13 years old so I'm not sure how bad it is for people with older domains/emails.