A plus sign is allowed in the user part of email addresses. Though it's most commonly used as a tag (and I've only seen it used that way) it could be used as a

A plus sign is allowed in the user part of email addresses. Though it's most commonly used as a tag (and I've only seen it used that way) it could be used as an actual address and since the receiving mail server decides how to handle it you have no way of knowing. If you choose to strip the tag I'd only do so when processing a new signup and make sure that the user can login with his user+tag@example.com and that all email goes to that address.