Strangely, I encountered a (possibly ajax related) bug relating to custom domain email addresses. I typically use a non--major email address for privacy reasons (AKA: not gmail, yahoo, etc) setup through ProtonMail using one of a number of domains I own. Interestingly the sign up form does not recognize this as a valid email address. I already had this account, so I logged in to post the report.
The domain ends on .com
That one is still pretty new, so that could be causing the issue. Do you have any email accounts with a domain that uses a more traditional TLD?
No issues here. What browser are you using?
I assume by doesn't work, it has orange text saying enter a valid email?
I guess the problem might be related to the general format of the e-mail address you entered. While the forum here supports "-" and "+" in the local-part, you might be using some other characters that aren't okay.
Are you serious? .com exists for over 30 years now. 
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Correct. It gives the Orange text. No unorthodox characters involved. Does it have a minimum char limit for part before the @? Mine is me@
Or possibly "international" characters like umlauts, etc.?