In general, the only way to contact someone privately, is to check their page and see if they have any contact details. If they don’t, that usually means they don’t want to be contacted privately.
For egsample, someone has kindly added a link to my collection in their collection and I wanted to thank them.
I would advice against that, especially on personal e-mails. In a lot of countries, a company cannot initiate contact with customers without consent. I’m not sure if that’s also the case for indies, or people trading as individuals, or how that works on an international scale.
There is a way to contact every user that has purchased one of your project from “Edit Game” -> “Interact”, but any attempt of communication has to go through an Admin before it gets send to customers.
At the end of the day, that’s your decision to make, hope this helps :)