I just debug it. The sender is set after the addon put items to the mail. However, the currentReceiver will change if we put the items to the mail.
The scenario of this bug is:
currentReceiver=alt1
sender="working"
put the items which should be sent to alt1 to the mail
currentReceiver=alt2, because the addon already put items to the mail
"Send All" button may send items to wrong character.