Why would you?
The only feature you should be interested in is if it's going to stay up and not disappear taking your emails and contacts with them. Any talks about encryption and not being able to read them should be taken with a grain of salt and at best, a bonus, not your first line of defense. When using email for DNM (or illegal in general) activities you should assume they are reading your emails, being hacked, logging IPs and are literally the FBI, just like the markets. Basically do these 4 things and you're golden.
PGP encrypt anything remotely sensitive
Use tor and preferably the sites onion
PGP sign all messages
Don't talk about yourself/life.
And just like that the issue of whether they are safe goes out the window. I just signed up for cock.li (more specifically cocaine.ninja lol), the same email grams uses, his real identity is known and he will comply with all warrants and fully admits it. Why is this a good thing? No false sense of security as I know I have to watch what I say, but I also know the site isn't gunna shut down suddenly due to LE pressure or intervention and I know it's not run by an amateur and riddled with bugs/exploits.
If you're using email for illegal activities properly how trustworthy they are shouldn't really matter.
Better yet, screw emails and switch to XMPP or bitmessage. Follow the same rules as before and you're double golden. Just my two cents on the matter.
lmfao i use google incognitus + gmail. I'm so woke and anon doing that.