If what you need is a guarantee that your data remains only in the EU, we don’t have that, and we’d rather tell you directly than let you assume otherwise.
Is there an alternative that really keeps data in the EU? (And not only in the sense it serves a sales promotion)
Depends on the definition and your threat model but to make a very large story short; it’s email, others have copies (your gmail friends?). Metadata is public by default the body can be encrypted and encrypted at rest (comes with many limitations) and that’s the highest level of security you can realistically achieve.
If that works fine if not, use another method of comm. Email wasn’t designed to be secure.
I do not know any EU-only, but ProtonMail is in Switzerland.
https://mailbox.org/ Germany https://posteo.de/ Germany https://runbox.com/ Norway https://www.migadu.com/ Switzerland
more: https://european-alternatives.eu/category/email-providers