I finally got around to setting up the e-mails correctly for my instance (mander.xyz) and noticed that the e-mail deliver-ability is awful, as I get heavily penalized by spam filters for having an .xyz domain. I would not be surprised if search engines penalize this too.
It is not a simple task to change a domain name for an instance without breaking federation, so once you have a federating instance you are are stuck with that domain name.
I would not recommend picking an .xyz domain for your instances unless you intend it to be a private instance forever. If possible, try to go for a .com, or some other more standard TLD!
There are email providers who will block email from brand new domain names, and perhaps others will consider it spam, and maybe only consider it spam for the very first email contact between your instance and their email server. Your results may be better within a few weeks time.
I hope that’s the case! I will run more tests in a few weeks.