Link: https://f-droid.org/en/packages/com.vayunmathur.calendar/

• add or modify calendar events and send email to guests without owners' knowledge

This app can add, remove, or change calendar events on your phone. This app can send messages that may appear to come from calendar owners, or change events without notifying their owners.

Sounds concerning but it could also be benign:

Add or modify calendar events

Likely used to show events you've been invited to (automatically like how outlook does it), and or update an event that has changed (cancelled, location update, etc)

Send email to guests [...]

Send email when adding contacts to and event.

That being said I don't believe the intent is malicious but I also haven't audited the code.

Without your permission, that's the part. The reason I even checked was because events were disappearing in my calendar for google. Birthdays were randomly showing and not showing as well. I absolutely think it's malicious.