PDF of the message on a diplomatic letterhead. 8.5x 11", 15 point type, 1 inch margins, seal of the State Department at the top of the first page.[1] The style is formal, but very plain and clear English.
Wars have started because of badly drafted or mishandled diplomatic notes. The Japanese diplomatic note announcing war was supposed to be delivered just before the attack on Pearl Harbor. The communications between the US and Saddam Hussein before the Kuwait war left Hussein with the incorrect impression that the US would not intervene if he invaded Kuwait.
I’m not sure in these specific cases, but they are frequently referred to as “diplomatic cables.” I don’t have much insight into how they are sent, delivered, or received, but they likely have formal agreements with host nations that specify how these cables are presented. Due to the sensitive nature of the communications, many of these messages are delivered in person, whether on paper or verbally, one diplomat to another. Electronic means of communicating cables are likely classified.
Animats in a sibling comment has the details. See section i:
> i. Delivery from the Department to Foreign Embassies in Washington, DC[…]
Executive Secretariat InfoLink seems to be doing the electronic heavy lifting.
Wars have started because of badly drafted or mishandled diplomatic notes. The Japanese diplomatic note announcing war was supposed to be delivered just before the attack on Pearl Harbor. The communications between the US and Saddam Hussein before the Kuwait war left Hussein with the incorrect impression that the US would not intervene if he invaded Kuwait.
[1] https://fam.state.gov/FAM/05FAH01/05FAH010610.html