A normal Ad Blocker does not block advertisements in mail (Spam Mail), the mail client you use must handle that. But advertisements on websites can be easily removed by an ad blocker, to provide a faster and less disturbing experience.
Ad Blocker can basically see the websites you visit. That way they can scan and remove advertisements. So it's a matter of trust. As I wrote it in another post where someone asked the same thing. Then you have to choose whether you want to share your information with everyone, or just a single company that you trust.
I myself use Ad Guard, both on my MacBook and my iPhone, and have never experienced any problems. Ad Guard is one of the major providers of Ad Blocking software and they get good reviews.
Ad Guard can be downloaded from the App Store and is free for iPhone. If you want to use their extended features, then it costs a small amount. You can also download and use the Ad Guard add-on for Safari on Mac for free.