Here's a good summary I found. It's a telegram service, basically.
Message Station
House Sivis operates message stations throughout Khorvaire. All cities and most towns have message stations; other than those founds along the lightning rail, most small villages do not have message stations.
For 5 gp, a person can send a short verbal or written message from one station to another, where it can be picked up or delivered. Message stations typically offer translation and notary services as well. If a Speakers Guildhouse is not present in the area, message station employees can arrange for other Sivis services, such as bookkeeping, mediation, and legal advocacy.