For anyone unaware, a community's outbox typically contains the last 50 or so Post Announcements - it is retrieved when you are the first person to find a community on a remote instance. It also seems to be fetched whenever an community on a remote instance realizes it's out of sync with the community on its host instance.

Compare:

curl --header 'accept: application/json' https://lemmy.world/c/memes | jq -r .outbox           
=> https://lemmy.world/c/memes/outbox                
curl --header 'accept: application/json' https://lemmy.world/c/memes/outbox | jq .       
{
  "error": "unknown",
  "message": "Record not found"
}

with

curl --header 'accept: application/json' https://lemmy.world/c/linuxmemes | jq -r .outbox      
=> https://lemmy.world/c/linuxmemes/outbox          
curl --header 'accept: application/json' https://lemmy.world/c/linuxmemes/outbox | jq .orderedItems[0]   
{   
    "id": "https://lemmy.world/activities/announce/create/0223f939-aafc-4215-9c20-a3460c967f63" 
    ... (the rest of the most recent post from linux memes)
}

(I came across this randomly, so I don't know if 'memes' is the only community missing an outbox. Others I've tried have been OK though)