Speaking of drawbacks of posteo, the lack of DMARC/ARC is one. On the plus side they have an option to encrypt your entire mailbox, separately from PGP auto-encryption for incoming emails.
Mailbox.org looks like all round decent email provider in my experience.
There is ProtonMail and Tutanota but they lack IMAP support (without bridge in ProtonMail case).
Skiff looks interesting but I haven't used it yet.