@juliobiason From the ActivityPub point of view, other actors sends you messages by posting (http POST) to your inbox URL (server to server protocol), and you retrieve those messages by getting (http GET) from your inbox URL (client to server protocol).
Nothing is said about keeping messages in the inbox after getting them or other interactions like patch/put/delete. The ActivtyPub Remove activity could be used to remove messages from your inbox (the inbox is a collection, and you can Add or Remove objects them from collections).
You could move messages from the inbox to a collection to keep them or to a trash collection to remove them after some time. You could also define rules to automatically move things posted to your inbox into collections.
I will find the fediverse easier to follow if it used an interface like a feed reader (I'm thinking of Akregator notably) than the Twitter-like interface of Mastodon and Pleroma. Notably when following bots accounts as they tend to spam timelines.
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