This doesn't have to work exactly as stated, but there is no messaging in the MUD, so I've copied the following from the Discord:
messenger NPCs who hunt down whoever (with limits; I don't expect NPCs in city A to head for city B where X logged out, without srs compensation) and delivers message. You pay a few coins to send NPC off, and recipient pays a few more to get NPC to say the message.
mansa - Today at 10:20 PM
Can be mugged and info stoleb
Nice.
Siva - Today at 10:24 PM
messengers have at least a few levels of pay options. 1) simple. You pay coins based on distance to where PC logged off, messenger NPC waits, and when they log in, NPC says, "I have a message from __!" and gestures to indicate an expectation of coins. Offer enough coins, get message and name of sender. Next level up: attempted anonymity. This costs extra coins, but not as many as other options. At this level, you can send a message and you've paid the messenger to /not/ tell who it's from. This is the simplest level of this misdirection, and if the recipient i willing to pay as much as you paid extra, messenger will fail to keep you anonymous.
3rd level up, you tell the messenger to tell the recipient message is from a specific someone else. This costs at least twice as much as the last option I discussed, and will play out the same way - except that the anonymity bribe will be half what the sender actually paid (though since that was double plain anon, amount should seem similar/identical. At this point, instead of the sender name, you get the fake sender name. Check for one or more relevant people skills, and if your skills are high enough, you detect that the messenger is lying. You can them bribe the truth out of the messenger.
Maybe more, not sure.
Don't kill messengers or get caught stealing from them. Should be in-game conseuences for it.
HOWEVER stealth-based factions should be able to acquire a disguise, and impersonate a member of the messenger guild, or whatever.
Yes, messengers can be killed - but you better have really needed that message not to get where it was meant to go. But if you are caught stealing from them, they will draw police attention, whatever police there are.
Everybody needs messages delivered, so the messenger guild has deals with all police.