I've been using various emails on a personal domain for different sites. Some time this year Six Flags put in an email check and I can't register my season pass

I've been using various emails on a personal domain for different sites. Some time this year Six Flags put in an email check and I can't register my season pass anymore. They give the following error: We were unable to finish validating your connection with the "*.net" email server. Yet I've been signed up for the pass holder announcements and get the email on a regular basis.