Hi pkg-kolab team, As describe last year in this bug, this problem comes up again. I am available to troubleshoot on this topic. 12D3261199: to=<user at domainx.com>, relay=kolabmailboxfilter, delay=0.78, delays=0.33/0.01/0/0.44, dsn=5.3.0, status=bounced (input/output error)