I've just tried to send an email to a data extension of 572 people. I created a new publication list for this.
When I did a send preview, in the part where you choose which recipient to preview the send of, a lot of the entries appeared as 'Invalid Recipient'. And then sure enough when I did a guided send of the email, using my new publication list, it only sent (and added to the publication list) 222 of the 572 people. I also notice that those people haven't been added to All Subscribers either.
Does anyone know why this could be?