I think the information security and logistical questions involved in raven messenger systems are really fun to think about.
For example, in Davis I ACoK, Stannis sent every raven on Dragonstone (117) to a widely dispersed group of lords to spread the accusation of incest. A lot of people know about it and are talking about it by the end of the book, implying a forwarding mechanism from those who got the letter either gossiping about it in their own letters to their neighbors or duplicating the message and passing it along. Stannis also has the letters delivered to the coasts en masse by ship, which could be more reliable but slower, and is limited by access to waterways. People use messengers carrying written messages on horseback and envoys to deliver messages verbally to convey things more personally or securely.
As you said recently, if you want to get a message to a specific lord, but you don’t have a raven for their seat, you’d have to get it to someone who probably does have a raven designated for them.
All of these methods and their lack of certainty make for interested information security questions. You can’t know for sure who among your contacts will have their contact, so a message may be lost not to weather or the dangers of the road but to being bounced around from castle to castle. You can’t be certain of passing information in any concrete window of time. You can’t know for sure your message is forwarded unless your intermediary gives notification of receipt and forwarding. Your message may have never been received unless you get a response.
Your seal is an attempt to prevent breach of secrecy between you and recipient, but it could be forged or carefully jimmied by someone other than the addressee. As we see in Davos IV ADwD and with the Karstark/Dreadfort maester with Stannis, maesters can’t necessarily be counted on to be neutral or loyal to their lord or seat. If the lord you are sending to is illiterate or functionally illiterate, or if you are, you’re dependent on whoever’s taking dictation/reading your letters out loud to do it faithfully.
In the Information Age, it’s hard to grok the uncertainty inherent in these forms of communication. From a worldbuilding perspective, it’s really interesting to consider ciphers and security measures to counter these issues, and the way that personal authentication like sending a ring known to be yours along with a message become important cultural factors.
But, like, in addition to security, there’s also the question of the simple length of the message. You can send a short note easily, even a full page is probably not a problem – but long messages with multiple pages? Ravens can be pretty large birds, but every additional page makes it more difficult for them to fly, and less likely for the message to arrive. Big detailed maps aren’t getting sent either. So besides ciphers and security seals, if raven messages were fully worldbuilt you’d expect the maesters to develop a compression algorithm, to fit the most words and images possible in the smallest amount of paper possible. (Hat tip to Terry Pratchett and the way he worldbuilt the Discworld’s clacks system.)