Is it okay that I buy an Email list? Its from a reliable source in our niche. And if yes then what should be the frequency for mailing them?
If you are certain that the source is reliable then go ahead many people do this.
Keep it civil 2 or 3 times a week or once a week is best.
I am doing this for the last 5 years so yeah you can buy an email list.
And about the mailing frequency I will say that don't overcrowd them with emails and also don't make them forget you. Twice a week is fine but the timing of the emails are important and its upto you.
It solely depends on what kind of list we are talking about here. Many lists in many niche needs 2-3 newsletters a week other need more and some other needs less.