Lots of logentries for propfind /exchange/<email>

[vanlier]

Recently I got warnings that /var/log was filling up on our Plesk 12.5 Linux server. I found out that about every 5 seconds there was a logentry for propfind /exchange/<email> with statuscode 405. Surely it has something to do with an autodiscover or ActiveSync on a mobile or tablet or something. The emailadresses involved belong to a customer of ours whose domain is on our plesk server. The mail and autodiscover records are pointing to their onsite server however, so why are the requests in the log even arriving at our server?