As I've started hosting more websites (mostly academic projects like unfold.studio or learning-machines.net), I have come to appreciate more and more the ability to point multiple domains' MX records to my single fastmail account. A nice additional benefit is the ability to handle wildcard email addresses. When I log in to ebay, it's with ebay.com@<subdomain>. When I log in to the IRS, it's with irs.gov@<subdomain>. When someone sells an email address or starts spamming it, I can cut it off. And I can hope that maybe a few less rows are getting joined in the big database in the sky.