> While I used spamassassin (not simple), my friend used Thunderbird which did a good enough job.
Err, what? SpamAssassin is a filtering system and framework, Thunderbird is a MUA. There were plugins to use ApamAssasin in Thunderbird, and there may have been some built in filtering in Thunderbird, but you'll have to do a lot better than just saying it was good to convince me it existed as anything other than a joke.
My job during the time Gmail launched and the years following was administering portions of an ISP, including mail servers and a separate set of SpamAssassin processing servers, as well as separate consulting work setting up SpamAssassin and a mail server for colocation customers.
I also used Thunderbird off and on for a number of years during this same period (when it wasn't Thunderbird, it was Mutt). No MUA without something like SpamAssasin (or some paid for service or package) was even passable during this time period, to my recollection.
> In the early Gmail days, I didn't know anyone who used Gmail for anti-spam use. It was all about:
Believe me, they did. People noticed it. ISPs noticed it. The ISP I worked for thought about shifting over all email for our customers to hosted Gmail, because the spam solution was actually better than SpamAssassin usually could do and it would offload a lot of work. It didn't happen, but it was at least thrown around as a possibility.
> Most average people I know did not care about spam during those days.
My experience fielding support requests during this time period points towards the opposite conclusion. People cared a lot more, it was much more of a problem then. Google trends backs this up.[1] It was bad enough that legislation about spam was being passed during this period, such as the CAN-SPAM act of 2003.[2]
I think perhaps you've forgotten just how bad it was, or were lucky enough to have an address that wasn't very badly affected. You were running SpamAssassin, so you would likely have been one of the better protected (and you could tweak your own rules, which might have helped if you did so).
1: https://trends.google.com/trends/explore?date=all&geo=US&q=s...
2: https://en.wikipedia.org/wiki/CAN-SPAM_Act_of_2003