Zimbra's IMAP server appears to send responses to a "SELECT" command that don't have any human-readable text at the end. So if a mail client attempted to select a folder named "MyFolder" the conversation might go something like this:
TAG SELECT "MyFolder"
* 1 EXISTS
* 0 RECENT
* OK [UIDVALIDITY 123]
* OK [UIDNEXT 456]
* FLAGS (\Answered \Deleted \Draft \Flagged \Seen)
* OK [PERMANENTFLAGS (\Answered \Deleted \Draft \Flagged \Seen \*)]
TAG OK [READ-WRITE] SELECT completed
Note specifically the line "* OK [UIDVALIDITY 123]"
While this seems to work with robust email clients, it's not quite up to the IMAP4rev1 spec (rfc 3501).
From rfc 3501's grammar:
resp-text = ["[" resp-text-code "]" SP] text
(where resp-text-code is "UIDVALIDITY 123" in this case.) The trailing space and human-readable text are actually required. The same applies to the UIDNEXT and PERMANENTFLAGS lines.
Most mail servers comply with this by sending responses along the lines of:
* OK [UIDVALIDITY 123] UIDs valid
Sorry I don't have server version details (or a support account in which to properly file a bug) since I don't run the Zimbra server I discovered this on.
Thanks for your continued attention to detail!