No, not entirely. Posteo uses at least strong TLS (transport route encryption), meaning the message can hardly be intercepted between your (posteo) mailserver and the recipient mailserver. Everything else is unencrypted: the emails rest unencrypted on the posteo server (unless you activate crypt sorage, easy to do) and on the recipient server (e.g. Gmail). The latter will analyse your emails even if you never consented. All this can be stopped using Posteo, though. They provide a lot of tools help on this on their homepage. Look for PGP and inbound encryption.