Normally, I would say: Never use such a domain name
grepnik.default.grepnik.uk0.bigv.io .
Nobody want's to use that name.
But:
D:\temp>nslookup 46.43.2.88
Name: grepnik.default.grepnik.uk0.bigv.io
Address: 46.43.2.88
So the reverse ip is that domain name.
A mail server should have such a configuration.
Conclusion: If you can't change the reverse ip of 46.43.2.88 to one of your shorter domain names: Use that long and terrible name.