Unable to delete messages

The logs were perfect. Here's an example with all the irrelevant parts cut out:

22:05:05 C: H4 SELECT INBOX (CONDSTORE)  
22:05:05 S: * 8 EXISTS  
...
22:05:05 C: H6 UID FETCH 1:* (UID)  
22:05:05 S: * 1 FETCH (UID 256298)  
22:05:05 S: * 2 FETCH (UID 256643)  
22:05:05 S: * 3 FETCH (UID 256666)  
22:05:05 S: * 4 FETCH (UID 256667)  
22:05:05 S: * 5 FETCH (UID 256668)  
22:05:05 S: * 6 FETCH (UID 256669)  
22:05:05 S: * 7 FETCH (UID 256670)  
22:05:05 S: * 8 FETCH (UID 270818)  
22:05:05 S: H6 OK UID FETCH completed  
...
22:05:06 C: S15 IDLE  
22:05:06 S: + idling  
...
22:05:49 C: J42 UID COPY 256666 Trash  
22:05:49 S: J42 OK [COPYUID 1 256666 270886] UID COPY completed  
...
22:05:49 C: K44 UID STORE 256666 +FLAGS.SILENT (\Deleted)  
22:05:49 S: K44 OK UID STORE completed  
22:05:49 C: K45 UID EXPUNGE 256666  
22:05:49 S: K45 OK UID EXPUNGE completed  
...
22:06:01 C: C94 SELECT INBOX (CONDSTORE)  
22:06:01 S: * 8 EXISTS  
...
22:06:01 C: C97 UID FETCH 1:* (UID)  
22:06:01 S: * 1 FETCH (UID 256298)  
22:06:01 S: * 2 FETCH (UID 256643)  
22:06:01 S: * 3 FETCH (UID 256666)  
22:06:01 S: * 4 FETCH (UID 256667)  
22:06:01 S: * 5 FETCH (UID 256668)  
22:06:01 S: * 6 FETCH (UID 256669)  
22:06:01 S: * 7 FETCH (UID 256670)  
22:06:01 S: * 8 FETCH (UID 270818)  
22:06:01 S: C97 OK UID FETCH completed

It is exactly as you describe. The mailbox is synchronized and a message is deleted (first copied to Trash and then expunged in the INBOX). There are no errors when doing this. Later, when the INBOX is synchronized again then the message reappears (UID 256666). My guess is that duplicates are created every time this happens in Trash, but maybe the copying is also ignored.

This appears to be an issue with the Zimbra server and I don't see any way to work around it. You can refer your IT department to this ticket and let me know if they have a different explanation.

It is most likely that using a different IMAP email client would reveal the same result, but maybe it's worth a try. If a webmail-client is available for the account then it might access emails directly (without) IMAP and that might work.