From, name and reply-to

• From address — who the message is from. Must be one the mail service will send as.
• From name — what a customer sees in their inbox. Empty leaves it as "WordPress".
• Reply-to — optional, for when replies should go somewhere else.

Two switches decide how far this reaches:

• Use this address for every message — on, every plugin on the site sends from your address. Off, a plugin that sets its own keeps it, and the mail service may refuse that one. On is the right default for almost everybody.
• Use this name for every message — off by default, because a shop that names its own sender on order emails usually meant to.