Here is a blog post I wrote on incoming email a while back,
www.toddklindt.com/.../Post.aspx?ID=31



Incoming email just matches email addresses if you use permissions.
You'll just have to make sure that the From address  of the meeting
request matches the email address to someone that has permission to add
items to that calendar  list. Remember that calendars have to be sent
meeting requests for them to show up properly.



If it doesn't work the first place to look when troubleshooting is the
SMTP server log on the SharePoint server. Verify the message is making
it from Exchange to SharePoint. Next check your SharePoint ULS logs to
see what SharePoint is doing with the message when it gets it.
SharePoint will not send any kind of NDR if there's a problem, so the
only information is in the ULS logs.