Spent the last couple of days working on this with PHPMailer I couldn't get this working with buttons enabled, kept giving me the dreaded "As the meeting organizer, you do not need to respond to the meeting". Without buttons however it worked fine.
It gave me an iCal attachment but nothing else in Outlook (although Googlemail worked fine!).
I eventually wrote my own script simply to handle iCal attachments, the problem seemed to stem from how the email headers were generated. I'm not sure how much direct control PHPMailer actually gives you over that. Anyway, I wrote it all up if anyone gets here, gets stumped like me and wonders where to turn next. Hope it helps and thanks xenophobia for at least helping me along the path a little -> iCal PHP Email Script
