When user receives a meeting request and responds to it (accept, decline...),
it's correctly added to callendar, but he can't send reply. Even the window
with options, if he wants to send reply doesn't appear.
This happens to all users in the company, but only sometimes. If I send 2
identical meeting request, the first is OK and the other has this problem.
They are sending emails through POP3/SMTP account, not Exchange, but still
using Exchange to store mailboxes.

Thank you!