"Read message" sticks in local outbox

When I send a new message, it goes to my local outbox and then is transferred to my mail supplier via a POP account. Afterwards, the message disappears from my local outbox and shows up in the local “Sent items” folder.

However, when the message is a “Message read” mail to the sender who requested such an acknowledgement in his mail, the message sticks in the local outbox and is not sent at all.

How can I sent a “messsage read” message to someone whose message I have successfully read?  Note: I have set my policy on sending “message read” messages such that I am asked by EmClient if I want to send it. If I click on "send “message read” message, EmClient generates such a message in my outbox, and exactly this message sticks there forever. 

Please help.