The phone is the HTC StrTrk, sold in the U.S. as the QTek 8500, mine is branded as the Dopod S300 (but it's the same hardward).
Phone's operating system is Microsoft Windows Mobile 5.0, and the actual client is the built-in message application (Pocket Outlook, I guess it's called).
The Zimbra Mobile feature (syncing Contacts and Calendar via ActiveSync over-the-air) works great. The IMAP mailbox I created on the phone syncs multiple folders properly, and quite fast (I use Edge). When I create a new message, and hit the "send" soft-menu button, it goes into the "outbox" folder.
Where it gets wierd is when I then do the "Send/Receive", Pocket Outlook gives me the standard error message "The message(s) could not be sent. Verify that you have network coverage, account information, etc., etc".
But - on subsequent send-receives, instead of that message, I get the message from "System Administrator" saying "Invalid message recipient", body = "Your message 'the subject i used' was not sent and has been moved to the Drafts folder. The following recipients are invalid:
somemail@otherserver.com"
I have "cleartext" login enabled on the server. Whether I use ssl on the client or not, the same thing occurs (1st attempt gives me the error message, subsequent attempts put a "Invalid message recipient" in my mailbox).
*** I AM using the default self-signed cert, which I recreated following the wiki instructions after the original one expired. ***
I'm going to SSH into the server and examine the logs next to see where the failure is happening exactly, and post them here. Although if ZCS 4.0 goes GA before I figure it out, I'll just switch to using ActiveSync.