Gmail will sometimes refuse email mentioning a helioho.st or heliohost.us domain. This is because we also have users who try to host illegal stuff like phishing. We remove the abuse as soon as we find it, but we have no control over gmail’s decision to block our domains anyway.
If this is an issue, I’d suggest using your account with an external domain like one from eu.org instead. Their domains tend to be more reputable as they’re manually reviewed when assigned, which helps stop people from registering abusive domains.