1. Sending too many emails
Believe it or not, there is actually a limit on the number of emails you can send. It is very high though, so most of us need not worry. if you try to send one email to hundreds of people at once, it can trigger alarm bells on Google's side. That's because it is a sign of a spammer.
2. Sending too many emails to addresses that no longer exist is another red flag.
Similarly to the previous one, sending too many emails to addresses that no longer exist is another red flag. This is again another indication of a spammer, as they tend to send loads of emails at once without any care about whether the addresses are active or not. So if you send too many emails to non-active addresses which result in a bonce back message, you could be mistaken for a spammer too.
3. Sending anything illegal
It goes without saying that if you're sending illegal stuff, your account could be banned. This can range from selling drugs and firearms, to distributing music and movies. While Google can't read your emails, it does use some AI detection tech to spot anything suspicious.