I have been using this email app for months, with 4 different email providers. It works like a charm, it's snappy, and privacy-oriented (preview links before opening them, block pixel tracking, and lot of other clever features.)
Oh and the developer is very active (several versions a week), very kind and answers very quickly.
One of the best bargains. (I bought the full version)
I noticed this feature in the default macOS mail App too but I'm not quite sure I understand correctly. If you click on the little arrow next to a link, it opens the website in a little window -- from a privacy/security POV, is that really helpful? Wouldn't the sender know you read the E-Mail if my computer sends a request to their server using that URL, no matter if it's for a preview window or my actual browser (except the latter has cookies etc)
In FairEmail, the "preview" is to show you exactly what URL is about to be opened, and give you the chance to force it to HTTPS, or remove known tracking parameters from it.
It also warns you prominently if the link's text differs from the target address, and allows you to pick which you actually want to visit (good for newsletters that mangle links for click tracking, but which show the raw link in the text).
Tapping a link will pop this box up, without the site being alerted, giving you the chance to decide whether you want to actually visit it or not.
Oh and the developer is very active (several versions a week), very kind and answers very quickly.
One of the best bargains. (I bought the full version)