Not irrelevant because the impression I had was that the majority of information (and orders) traveled via the standard x-boats.
That's what the story about how Norris got advance notice of Strephon's death says, yes. Unfortunately, the story is starkly unbelievable, precisely because of the existence of navy couriers[*]. In addition to Norris, at least 34 fleet admirals (plus any dukes said admirals might feel it their duty to inform) would get the news just as fast as Norris.
So if we ignore that story, what have we left? An X-boat network that has not been updated to jump-5 in 700 or to jump-6 in 1000. Navy couriers capable of jump-6. The only explanation for why the X-boats weren't updated to fulfil their function is that they had been superceded and made irrelevant. My take is that when jump-5 was invented, the Navy kept it a military secret for as long as they could. A couple of decades, perhaps. And during that time, the Imperial bureaucracy got into the habit of sending copies of reports and orders by navy couriers. No doubt the originals were still sent by X-mail, as the regulations bade; indeed, I'm sure the originals are still sent by X-mail by the Classic Era. But when the J5 drive were finally declassified, there was no urgent need to update the X-boats, and they lost out in the budget fights. Almost exactly the same happened when the jump-6 drive was invented.
[*] And also because if the Emperor can have a secret jump-6 courier network then so can archdukes, dukes, megacorporations, and high-population worlds. Maybe not as extensive as the Emperor's (though the megacorporations might well have), but certainly good enough to convey such momentous news as the death of an emperor.
The Jump-6 x-boats are a secret and the only way to keep something like that secret is to use them judiciously. If every single order to the outlying regions was transmitted via jump-6 ships (while regular mail was transmitted via jump-4) there would be so much jump-6 travel that it would be very difficult to keep it secret.
The Emperor's courier network is secret (As secret as the existence of any intelligence agency, anyway). The navy couriers are public knowledge. There are a couple of TNS newsbriefs datelined on Regina that cites dispatches brought from Terra by Navy courier. They not only demonstrate the existence of jump-6 couriers beynd any doubt, the two runs are so fast that conspiracy theorists must be using them to prove the existence of jump-7!
Of course with the model of the jump-6 network that I mentioned above you wouldn't be able to do that because I wouldn't assume that each station would have an available jump-6 boat to be placed on standby. They probably would jump in, transmit their much smaller information package and get refueled and sent on their way.
Jump-6 couriers aren't that expensive and any fleet admiral will want a whole lot of them to manage his fleet in wartime. Using them for courier duty seems a natural way to employ them during peacetime.
If the next boat is able to jump within 6 hours then there probably isn't much point in multiple ships. Cutting out 6 hours per jump would make a difference of 10 days to the furthest reaches of the Imperium. So you would go from 294 days to 304 days, reducing transmission time by about 3% while doubling the number of ships needed. Likewise you probably would not send multiple ships just so that the first one that arrived could pass on the message faster. Your average time savings in that case would probably be less than 3% while you've at least doubled the number of ships.
That's why I mentioned them as an afterthought. Of course, if advance notice is as valuable as the MT text says, 10 days can represent vast fortunes.
Hans