We are encountering a weird problem. When we reply an email from a shared inbox with sent on behalf of check box ticked, we won't be able to sent out this email as Zimbra will prompt Permission Denied.

However, we overcome this problem by either removing the image/logo from our email signature, or save the email to draft first, then reopen the same email from draft to send.

Any idea what cause such a weird permission denied issue and anyway to resolve it?