For the record, the usage of the plugin with a Microsoft Exchange server prevents to send mails to some recipients. The reported error type is MapiExceptionNamedPropsQuotaExceeded.
The problem is due to the presence of UserProperties in the mailItem when sent to those recipients. Note that not all recipients (within the same organization) are affected by this problem.
The issue is actually already solved in build >= 3667.
See also http://support.microsoft.com/default.aspx/kb/948856
For the record, the usage of the plugin with a Microsoft Exchange server prevents to send mails to some recipients. The reported error type is MapiExceptionNamedPropsQuotaExceeded. The problem is due to the presence of UserProperties in the mailItem when sent to those recipients. Note that not all recipients (within the same organization) are affected by this problem. The issue is actually already solved in build >= 3667. See also http://support.microsoft.com/default.aspx/kb/948856