encrypt inbox without uploading private key

Hi,

I have just started with Mailbox on the free trial but I'm stuck on the encryption options. I neither want to use Mailbox Guard where Mailbox manage the security, nor Mailvelope with a browser extension.

Instead I just want to create a PGP pair myself, update only the public key, and decrypt emails using my private key when they land in my local imap inbox (forgoing the use of webmail).

I've tried to set this up, but I'm still seeing new emails in plaintext in the web client (which should not be possible if this was working) and I'm also able to receive emails over IMAP without using my private key.

I've gone through the 'Security', 'Email encryption' and 'mailbox Guard' options and cannot see how to do this. Is it that I can't do it on the free trial, or does one have to use Mailvelope if not uploading private key?