StartMail encrypts all of your stored mail by default, which is a better baseline than the options below it. The catches are that it is closed source and its encryption runs server-side rather than zero-knowledge. You are trusting how it holds the keys. It also keeps an encrypted copy of deleted mail for a few days.

StartMail
Dutch email that encrypts all stored mail by default.
startmail.comPaidWebNetherlands
Pricing$29.95 for the first year, then $59.95/year. 7-day trial, no free tier.
Strengths
- All mail encrypted at rest by default
Considerations
- Closed source
- Server-side encryption so not zero-knowledge
- Keeps an encrypted copy of deleted mail for a few days
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