One way that I find emails for the domain owner is to use Whoxy.com. What I am about to suggest doesn't always show the email address but in conjunction with some of the ideas mentioned above should at least help increase your chances of finding a contact email address, especially if the domain is not in use and you can't find a business email address.
Now I know a lot of you probably already make the assumption I am about to outline, but it's for those of you who might be unsure how to double check before contacting the person's name you may find in older Whois entries.
So what you do is complete the Whois check on Whoxy and Whoxy will give, in most cases, the historical Whois owner data. So if you're lucky you will see the owner/registrant info, if they had made it public before the GDPR kicked in and most domains went under privacy.
In order that I am confident that the domain under privacy was owned by the registrant, whose details are showing from the previous years Whois data on Whoxy, I look at the Namerver info. and if the Nameservers are the same for 'all' the Whois entries, all the way up to the last Whois record then I make the assumption that the domain under privacy belongs to the person who's name shows prior to it going under Privacy.
Bear in mind there are some domain owners e.g. domain investors, where the Name servers may change around a bit as they try and sell their domains on different platforms and point the Nameservers to the sales platform Nameservers.
But in my experience as a broker looking for owners of domains, where the domain is owned by a small/large company or an individual who only owns a few domains, it's rare to see the Nameserver change for the duration of their ownership of the domain.
Let me give you an example.
Do a search on Whoxy(dot) com for business (dot) com and you will find a lot of historical Whois for that domain. In July 2017 the Nameservers updated to 'awsdns' and then in October 2017 a name/email appears for the registrant. The Nameservers remains at 'awsdns' from July 2017 all the way up to April 21 2020.
Based on that information I would make the 'assumption' that the persons name/email that appeared in October 2017 is still the current owner.
This is by no means fool proof but more often than not when a domain is sold then in 'most' cases the new owners will host the domain on another Nameserver.
But if the Nameservers don't update from year to year then it's likely to be in the hands of whoever showed up in the earlier Whois entry, assuming there is a name/email, as per my example.
And if it transpires the domain is no longer with the earlier registrant found then there's no harm asking if they know the name and/or contact details of the new owner.
If all else fails with Whoxy then go to Archive.org and see if there was ever a working website on the domain and see if you can find a name/email from the Archive entries.
