Their privacy policy seems fine at first but there's a few suspect things
3. What we collect
3.1 Account data
Required: your username, password (stored as a hash), and mailbox settings needed to operate your account.
Optional: anything you choose to send to support.
3.2 Email content
We store the emails you send and receive so the service can work. This includes message content, headers, attachments, and standard delivery metadata that exists inside the email itself (for example: sender, recipient, subject, dates, and routing headers included by mail servers).
Pretty good, for the last one that's just how IMAP works. But wait:
Allowing remote IMAP/SMTP makes it much easier for accounts to be abused for spam, and it can also create extra ways to link activity back to a user. To keep the service private and protect deliverability for everyone, we only support access through our web interface.
Why would they specifically try to prevent you from using a mail client? Seems very suspect to me. Doesn't seem like they support POP3 either which would prevent them from storing e-mail contents.
5. Sharing and third parties
We do not sell your data.
We do not share your personal information with third parties for marketing or advertising.
We may rely on infrastructure providers (for example: hosting, bandwidth, DDoS protection, and email delivery components). These providers may process limited data as needed to operate the service.
I wonder what counts as "limited" for the third parties?
7. Data retention
Mailbox data: kept until you delete it or your account is removed.
Billing records: cryptocurrency transaction details are retained indefinitely (see Section 3.3).
Abuse and security: if an account is used for abuse, we may take action including suspension or deletion. Limited records related to security incidents may be retained to protect the service.
Backups: deleted data may remain in backups for a limited time until backups are rotated.
Again, "limited" with no specifics.
Account deletion: only upgraded (paid) accounts can delete their account.
This is somewhat unusual too, albeit they allow you to delete your mailbox data at will (minus the "limited" time when its stored in backups, of course)
I don't know, maybe I'm being overly paranoid over some vague wording but I wouldn't trust them.