Hi Karol,
Good Day,
I hope you have the following things checked,
1. Telnet the port which you are specifying in the incoming mail server settings from the application server to the mail server.
2. Log into the outlook mailbox with the user name and password which is used in ServiceDesk Plus.
If you need assistance in setting up office 365 in ServiceDesk Plus then kindly send an email to servicedeskplus-support@manageengine.com with this forum as the subject so that we can help you.