The sender address is the inbox where reply emails will be sent if someone responds to your email. This is also where any “Out of office”-emails will end up. You choose the reply address you want to use.
Avoid using a gmail-, hotmail, yahoo-, aol-address etc as the sender address, since it will increase the risk of your email getting stuck in spam filters.
If all new mailings should have a the same sender address it is better to change the sender address for the whole account »