All email messages you have sent from “me” will be labeled with the Gmail Sent label. Gmail treats the email address of your account or any alias email addresses as “me”.

It is important to understand that the Gmail Sent label is not an actual label – it is a virtual label created by Gmail: all email messages sent from “me’. In other words any email sent from your account email address or from its aliases will be inside the Sent label.
So:

• It is impossible to add the Sent label to an email which is not sent by you
• It is impossible to remove the Sent label from an email message
• You cannot remove, rename or move the Sent label
• If you add an alias to your Gmail, then all email messages sent from that alias will be in the Sent label