Emails not sent while replying.

When I reply an email it doesn’t work. No error message, it just don’t do anything, not appears in sent folder and the other person confirmed he didn’t received it.

http://screencast.com/t/STSUUd4Uj6A

Hi,
could you right click the Smart folders and choose to Display > Outbox and see if the email is stuck in there?
Then please check Tools > Operations window for any errors and after trying to send a reply, please copy the content of the Log tab (also located in the Operations window).

Best regards,
Olivia