She completely replaces the vanilla male courier with a custom-voiced female NPC.
When she isn't busy sprinting across the tundra to give you a letter from a "Friend," you can actually recruit her to join your party. Lily - Skyrim Messenger and Follower
Do NOT set her home residence via other follower mods. If you do, she might get stuck there and you won't get your mail!. She completely replaces the vanilla male courier with
Tired of the same old "I've been looking for you. Got something I'm supposed to deliver—your hands only" from that scrawny vanilla courier? It’s time for a change. Lily - Skyrim Messenger and Follower