The old modmail system has been retired. Subreddit.mod.inbox, Subreddit.mod.unread, Subreddit.mod.stream.unread, SubredditMessage.mute, and SubredditMessage.unmute have been removed.
Use Subreddit.modmail.conversations(state="new") to fetch conversations. To stream, use SubredditModerationStream.modmail_conversations. Muting/unmuting is now handled via ModmailConversation.mute and ModmailConversation.unmute.
# Fetching conversations
for conversation in reddit.subreddit("test").modmail.conversations(state="new"):
print(conversation.subject)
# Resuming a listing using 'after' via params
conversations = reddit.subreddit("test").modmail.conversations(params={"after": "2gmz"})