Ranked in these QuestionsQuestion Ranking
Pro Numerous feature/advantages
Pro No ads
Posteo is ad-free with no ads when using their email services.
Pro Support for IMAP/SMTP
Posteo supports email clients such as Thunderbird and Claws Mail which will allow for greater control over encryption and user's private keys.
Pro No Personal Information required to register
Registration through VPN is allowed and no personal information is required, just a username and password.
Pro Very secure
Posteo does very well on Dismail's server list and it supports end-to-end encryption with PGP. This can be done in the browser or with an email client, and Posteo has a help section on how to set up encryption.
Pro Inbound encryption
You can select an option to encrypt inbound email from their server with a user-provided GPG key.
Pro Encrypted storage
You can select an option to encrypt all of your emails stored on their servers.