One of my users experiences a strange behaviour with his exchange 2010 account. When he invites another user within the same organisation/same server, the invite is shown as sent but the recipient never actually received it. To make sure it wasn't linked to his outlook profile, we did the same test using his OWA account. Same result. The meeting is created in his calendar, the meeting request is shown as sent.
There's no problem with emails. All emails he send are correctly received.