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Since Yosemite version 10.10.3 and 8.2 update to Mail.app, Mail.app's handling of sent items with Smartermail EWS has gone awry.
The expected behaviour is that after sending an email, a single copy is put in the corresponding 'Sent items' mail folder.
For some reason now, after sending an email, there are two copies that appear in the 'Sent' mail folder. One is marked 'read' and another marked 'unread'. This leaves a nasty unread mail count number in the sidebar alongside the sent items folder.
Sending an email from the same installation of Mail.app through an MS Exchange EWS account doesn't exhibit the same issue.