My point for that post was that it is important to have a agreed subset of the full character encoding allowed by the e-mail address specification. This subset helps to enable Interoperability, by having everyone make sure that they can send-to, and receive-from, an address fully encoded. This problem came up when some systems had trouble with the apostrophe, such as is needed (desired) for people names like "Fred O’Donnell". That is, a sending HISP didn't allow the user to send to a valid email address because it contained an apostrophe.
Security ConsiderationsSo as helpful as apostrophe is for people names like "Fred O’Donnell", it is problematic as it can be used in malicious ways. This because the apostrophe is the single-quote. Here are some general email address attack articles: