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!
Oh, absolutely.
I also tried setting up my e-mail server at home, but my ISP won’t let me forward port 25.
I think it is related to this. I also have a really high trust value an email tester and I use a .com. It doesn’t really matter. The email service we have on our company must have some kind of whitelist of “trustworthy” mail servers. I have sent emails to my company address and they go to SPAM. I’m pretty sure the whitelist is pretty small, or at least it doesn’t include email services provided by non big companies. I use Migadu, btw.