A user using Outlook 2010 sends calendar event invitations. Sometimes they are received properly, sometimes they are not received at all (although outbound spam filter that is outside of the network shows they were sent), sometimes they are received as regular messages with attachment "Winmail.dat". The recipients are using Lotus mail client.
I have checked on the sender's Outlook and confirmed that it is NOT set to send RTF, I also checked the individual contacts that they are not set to send RTF.
The sender seems to be sending them in the same manner each time. I would appreciate any help.
Thanks in advance.