It’s in California, and that’s where the data is stored. It’s not stored with end to end encryption, so I understand your worry, and unfortunately Port87 wouldn’t be a good choice for your needs if that’s not acceptable.
In the future I might make the system work with locally hosted databases, so you would still use my SMTP servers, but everything else would be locally hosted. That’s probably a while in the future though. I have some priorities before exploring that option.