Hello Dedalus,
I got your email and run some checks with the devs.
It looks like your client is sending a wrong header that causes the following exception:
Code:
2012-09-03 16:22:30,132 [ZxMobile Handler Thread] debug Base64 rawData: TWVzc2FnZS1JRDogPEVFNDRFOEU1LTEzODEtNDAyOS04RDVELUVFRDIyNTY0MEE1MUBmcmlnZWwuY29tPg0KRnJvbTogIA0KVG86IHJvb3RAZnJpZ2VsLmNvbQ0KU3ViamVjdDogUmU6IENyb24gPHJvb3RARkFQU3J2M$
2012-09-03 16:22:30,137 [ZxMobile Handler Thread] warn SmartReply: Unable to send mail: java.io.IOException: Unknown encoding: utf-8
The header causing the issue is "Content-Transfer-Encoding", which in the logs you sent me is set to "utf-8" which is not a valid value according to RFC 1521 (RFC 1521).
As stated in the RFC, possible values for the field are:
Code:
"7bit" ; case-insensitive
"quoted-printable"
"base64"
"8bit"
"binary"
x-token
A bug regarding this was opened on the Zimbra bugzilla in 2009 but it was closed as "WORKSFORME" as the issue is clearly on the client side (so must be fixed by HTC).
I'll talk with the devs to see if we can apply a workaround 
Have a nice day,
Cine
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