This is gonna be difficult for us to help with if we can't see the data (which you shouldn't share with us for privacy reasons). Did support specifically say the special characters causing the error were @ and ., and/or that they were in the email address column? You're right, those characters definitely should not cause a list import to fail if contained to the email field, because (obviously) that field does accept those special characters. I'd expect that it's likely that the error was caused by special characters present in another column - one that matched to a field type that doesn't accept them, like an integer field. You might want to do some filtering through the other columns in your spreadsheet to see whether they or other special characters are present in them.
Hope that helps.