the sender' own address if this header is enabled in mailconfigthis prefill sends copy to the sender (in addition to that written to the sent-mail file)but it can be deleted if unwanted also in compose windowsthe attach button issues file selection dialog for attaching file to your messageas in figure - the parts button pops up dialog displaying files already attachedlike that in figure - when your message is sentthe text in the edit portion of the window is sent as the main message textand any attached part files are sent as attachments properly encoded according to their type figure - attachment file dialog for attach as we've seensmtplib ultimately sends bytes to server over socket since this can be long-running operationpymailgui delegates this operation to spawned threadtoo while the send thread runsa nonblocking wait window appears and the entire gui stays aliveredraw and move events are handled in the main program thread while the send thread talks to the smtp serverand the user may perform other tasks in parallelincluding other views and sends you'll get an error pop up if python cannot send message to any of the target recipients for any reasonand the mail composition window will pop up so that you can try again or save its text for later use if you don' get an error pop upeverything worked correctlyand your mail will show up in the recipientsmailboxes on their email servers pymailgui demo