Re: Most of those red flags are complete nonsense
For a recruitment agent to communicate via InMail, they either need to be connected to a person (i.e. the person accepted an invite) or they have to spend an InMail credit. They only get a small number of credits per month and they get the credit back if the person responds within 90 days.
So I don't respond. If I respond then not only do they get their credit back but they'd probably think it okay to spam me again and again hoping I respond each time. I also ignore invite attempts by recruiters since I know they just want to be cheap with their credits - I soon learned that having a recruiter in contacts is a terrible idea.