Hi everybody! My first post here, so be gentle :P

We have a zimbra 8.0.5_GA_5839.FOSS install with a single domain with about 300 accounts. It is working quite well except for this issue we can't figure out:
some users have configured another account as POP (from same domain), when they write a new message with that identity, requesting read receipt, the message have the disposition-notification-to header half empty like that: " disposition-notification-to: John Smith < >"
When the receiver clicks to answer to the read receipt it gets an error like this:

method: [unknown]
msg: system failure: error while sending read receipt
code: service.FAILURE
detail: soap:Receiver
trace: qtp426168733-42597:

Body: {
SendDeliveryReportRequest: {
_jsns: "urn:zimbraMail",
mid: "1974"
Header: {
context: {
_jsns: "urn:zimbra",
account: {
_content: "johnsmith@mydomain.com",
by: "name"
authToken: "(removed)",
session: {
_content: 11654,
id: 11654
userAgent: {
name: "ZimbraWebClient - FF26 (Win)",
version: "8.0.5_GA_5839"

I also tried setting the "Reply to" filed in the account settings but with no change.

Sending from the main account works fine.

Where should I check ?