> For instance, a startup of 3 would not need to send 500 emails a day, that is a clear red flag.
There are many startups that do cold outreach, via cold emailing, that could do roughly 500 emails per day. I am not sure how typical this is, but as someone that has started doing sending cold emails as a direct sales tool, it seems to me that you just haven't had any customers like that yet.
If you do though, don't assume they are spammers.