Cal-Dav - Mass cancel meeting emails were sent

Hello, recently one of the email users who is using CalDav for a couple of years faced the following problem. A bunch of emails (around 150) were sent to meeting collaborators, regarding old meetings(some of them dating two years ago). The emails were proper Cancel meeting events with subject : "Cancelled : The name of the meeting" When we took a look at the email headers, it seemed that this was sent by "User-Agent: Kronolith H5 (4.2.24)" which is the HORDE webmail if i am not mistaken. This user does not use HORDE webmail for organizing meetings but only Outlook and a corresponding caldav plugin. Do you have any idea on why this could be happening? P.S. I am using CPANEL / WHM version v80.0.15. I know it's old. I plan to update in the next couple of days.