Yesterday it suddenly started to build a queue, the reason being "mail transport un available" and also "delivery temporarily suspended: conversation with omega.internal.citrix.com [18.104.22.168] timed out while receiving the initial server greeting". Restarting zimbra "fixes" the problem fo about one minute and then it reappeared. Because there is only one local account on this server I deleted and recreated the account and this "fixed" the problem for several hours only, then it started again and after several hours cleared itself with no intervention. This server has run without problems for years util yesterday. What to look for and where ?