It's not necesarily banned, but very often port 25 will be blocked. Additionally it's pretty likely your email will be marked as spam if it comes from a residential IP. They must have thought about this though, so maybe they have their own SMTP servers that emails go through.
"Helm connects securely to a unique gateway, which is assigned a static IP address so Helm is reachable by other mail servers and secure TLS sessions can be established."
Which I dont see the point of then. That way they are your gatekeeper of your mail/files etc. Not much of an improvement.
