When comparing ProtonMail vs Posteo, the Slant community recommends Posteo for most people. In the question“What are the best email service providers?” Posteo is ranked 1st while ProtonMail is ranked 5th.
Ranked in these QuestionsQuestion Ranking
Pro Privacy respecting
User data is protected by strict privacy laws because all servers are located in Switzerland.
Pro Built-in end-to-end encryption
Pro Open Source
Pro No personal information needed
To create an account you don't need to give any personal information, just choose username, domain, and password. Even the recovery email address is optional.
Pro Mobile apps
Apps for Android and iOS available.
Pro Overly Expensive for Desktop Support
Pro Numerous feature/advantages
Pro No Ads
Posteo is ad-free with no ads when using their email services
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 No Personal Information required to register
Registration through VPN is allowed and no personal information is required, just a username and password.
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.
Con Still lacks some useful features
At the moment, there is no calendar feature and contact management is quite poor. They will, however, start to work on it.
Con Ties to US Investors
ProtonMail takes money from US Investors
Con No IMAP and SMTP support for basic accounts
Basic accounts are restricted to the Protonmail web client. Support for IMAP and SMTP is available with a paid subscription only, and as such with basic accounts it is not possible to send and receive email with external applications.
Con Overly Expensive for Desktop Support
Con Requires phone number for login
If you sign up over Tor or a VPN, it will require email or phone number verification. Email verification is disabled if you use an email that isn't from Google or Outlook.
Con Encryption Keys are stored server-side
The keys are generated during account creation. Using your existing keys is prohibited and ProtonMail must store and control the private keys. Encryption cannot be secure unless the user controls the private keys.