Thanks for reporting this! It appears that the regular expression we are using to validate the email address is not accepting uppercase letters. Not sure how this slipped by, but I've just created a bug ticket for it here:
https://github.com/DSpace/dspace-angular/issues/1983I've also fixed it in this PR:
https://github.com/DSpace/dspace-angular/pull/1984. This fix should likely be in 7.5 (due in Feb). In the meantime, you could update that Regex manually in your codebase & rebuild/redeploy the User Interface if you need it sooner.