Here's something I think of implementing but I'm not motivated enough so I'm curious if others would like it too.
I find very unrealistic that the courier finds you wherever you are. Instead of that I'd change them to drop the letter at your last known location: the last player house you visited. He'll slide the letter under the door but it will give it to you directly if you by chance intersect his path. Variations: drop it at your house in the same or closest location from where the sender is; courier starts from the closest inn to the sender.
So, what you think?

Sounds like a cool idea. Implementation seems tricky though. Might simply result in a lot of undelivered messages. The problem is that you aren't expecting a message so you wouldn't go looking for it. I might be in Windhelm doing stuff, miss the courier so he stick it under the inn door. Then I leave and don't return to Windhelm for 10 levels.

I guess one solution to that would an npc finds you at the next city you visit and says something like "There was a guy (courier) here looking for you. I think he said he was heading to Windhelm"

Believe me I thought of this before you edited. Also I don't recall any letter to be crucial to a quest line, just for radiant quests. And won't be under inn doors, it will be under player house doors.
edit: You can get a popup when opening the door "You found a letter" and it will be added to your inventory, even in vanilla you could not read the letter for "10 levels" so the quest won't be started.

PS: this is something I may do if out of ideas or need a break from my current wips. If other modder likes it they can take it (but I reserve the right to remake it if I don't like the result)

Sounds good. Hope you decide to do it. I would definitely use it.

How would you go about supporting mod-added player homes? Or even the possession of multiple vanilla houses?
I've got quite a few, and it's not unheard of for me to visit any which one of them, including vanilla homes. In this case, if a message was delivered to the last house I visited, chances are I wouldn't find it for a loooooooong time.

What about if couriers could only find you in/near major cities? Or when leaving one of the many inns that populate Skyrim?

Player houses could be supported if the modder set the usual vanilla keyword "locationplayerhouse" or something. But you right, they may be to many and vanilla could be enough....one way to solve it could be to place a quest marker once an NPC tells you that someone looked for you.

Still questionable how he found me from my point of view.

Unless you roleplay a very incognito character, you tend to be a pretty high-profile person (especially if you are recognised as Dragonborn). I don't think it's outlandish then that word of your whereabouts would be gossip, or talked about. Maybe those couriers do the old Oblivion trick and have contacts in beggar society......

I don't think you can claim ownership over an idea posted on an open forum, and everyone is generally free to remake mods, but to use content from the original they'd need permission from the creator.

At level 81: "So you do what, fetch the mead?"

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