Just did some testing around this and I see that error message in my test instance if I don't set the .com portion of the domain in the setting. i.e. if I just type in gmail instead of gmail.com then I'll get that error. What should show an error message like Please use an @gmail.com email address instead of Please use an @gmail email address.
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However, it is entirely possible that the email domain isn't being read correctly. What I would suggest is go back to the domain setting (https://FQDN/admin/features) and type in gmail.com again then save. Regenerate the invite URL and try to sign up again. if you continue to get an error with the domain set correctly, please reach out to HC Server support at support.atlassian.com and we can investigate further.
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