It seems that MS Outlook Mobile doesn't handle calendars correctly...

I suggest you to try to use GMAIL mobile app and see if it works better

I want to try the Gmail app on my phone, but on my computer, I have no option other than Outlook. Also, the devices do not remind me 15 minutes before the event.

You need to verify in OUTLOOK that the in the calendar event it has the remìinder active.

If this is active and you not see it, that's a Outlook issue not SmarterMail...

It is active, but it doesn't work for any of the 150 users. I think something is missing.

I fully understood the issue after multiple tests.

The reminder is sent to the meeting creator but not to the other participants, even though the reminder is active.

This is a strange issue. How can it be resolved?

Just to circle back on this, I believe the issue being discussed is the "Email Notification" setting when creating a new appointment. (Correct me if I'm wrong.)

Regarding that, an email notification is only sent to the meeting organizer, not attendees. Our Help documentation states the following (https://help.smartertools.com/SmarterMail/Current/Topics/User/Collaborate/Calendar/CreateNewAppointment):

Email Notification - Enabling this will send an event reminder to the organizer's email address based on the time interval set by the reminder. This email is in addition to the notification in webmail or any synced client.

If a reminder is set for the appointment, then attendees will receive browser notifications or standard reminders in whatever client they're using.