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.

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,