Yeah, forwarding is an easy way to lose your email service. It's also a great way to get our server blacklisted for spam, since forwarded spam looks like we sent it instead of the actual spammer.
The typical way to resolve this is to just have Gmail log in and pull the mail from Plesk instead of using Forwarding. You can do that by turning off forwarding, then going into Gmail's settings and adding the Plesk mailbox under "Check mail from another account". If it asks for a server name after you enter the email address, use johnny.heliohost.org for the server (Plesk may suggest using your domain as the mail server, but that configuration is not supported on HelioHost). Doing it from Gmail's end will make your account use 0 emails/day for forwarding, avoid us looking like a spammer, and eliminate the issue with your mail limit.
Mail service has been turned back on again. Please fix the issue quickly. If you need assistance getting it to work from Gmail's end, let us know and we can assist. Many other users have done it that way.