@aral as Bob mentions I got a strong whiff of cryptobro when I saw they have a dedicated token site, and you buy usernames with tokens.
I am a user and proponent of Delta Chat (aka #chatmail I looked up Sesssion a bit and they're somewhat similar in concepts, but differ in implementations.
For example, Delta / Chatmail is based on relays (custom email server deployments enforcing encryption policies) whereas Session uses Swarms (5-7 nodes cache an ID range).