Getting: Unable to send attachment with Mail Drop

Mail Drop is not working for me, I always get error message while trying to send.

Here is what happens:

-I compose a message that includes a large attachment

-I click send

-I get the message "Unable to send attachment with Mail Drop" for a few seconds

-My message is not sent and it goes into the Outbox folder.

Configuration:

OS X 10.10 Yosemite

Mail -> Preferences -> Accounts -> Advanced. The option "Send large attachments with Mail Drop" is selected.