Information I researched to share
This is only blocking mail delivery from HelioHost to Gmail. No problem from Gmail to HelioHost.
I could not confirm HelioHost registration on the official blacklist. This issue only affects Gmail.
This also blocks emails sent by HelioHost to you. But don't panic if you've gotten HelioHost information outside of email!
If you are forwarding mail from HelioHost to Gmail, it will also be blocked. Use POP instead.
Presumably this block is a domain level block for heliohost.org and heliohost.org. If you are using a custom domain you may not be affected by the block.
If you receive a notification of Plesk migration in the future, if that email address is Gmail, that email is currently blocked, so please change the notification email address.
I had Gmail registered with HelioHost, so I was affected by this, but since I'm on Discord, I didn't have to rush to change my email address. Discord is the fastest for news and pinging me. If you're a regular user of HelioNet, don't panic as well, because there are alternatives to e-mail.
Note that some servers may have different restrictions. (Especially Johnny)