In you want to leave a copy of sent message in a folder "Sent" , You have to check "Save a copy in Sent" in the drop-down menu "More" in a compose form.
If the tick isn’t there then after sending a message it won’t be saved in your mailbox.
You can turn on or off this option in section “Options” - > “Name, signature & sent folder”.
You can also choose in which folder do you want to save sent letters.
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