Osnat Farouz wrote:

When I send a meeting request the recipient does not receive it as a
meeting request but as a regular mail. In Outlook options, the option
"when sending meeting request over the internet, use iCalendar
format" is checked.

Is that person in your Contacts folder? If so, open the contact record and
double-click the mail address. Examine the "Internet format" drop-down at
the bottom and make sure it says "Let Outlook decide the best sending
format".
--
Brian Tillman