Courtiers and Messengers of Persia:
They are the people who always deliver your messages, but they also have a more secret option. They are the ones who send anonymous messages for you. They require 5g as payment per anonymous message. When hired, it will count as if you delivered the message to their mailing headquarters unless you specify other way. They offer 1 messenger per person to be hired personally for 10g per turn.
A messenger takes 1 session-turn to deliver three letters in one city. Remember that all units travel the same way.
Investigator's Guild of Persia:
They are the very demanded investigators you always hire for 20g. When hired, it will count as if you traveled to their guild to hire them, unless you specify another way.
Nitpick for the newcomers: I assume these should be changed from Persia?