The number of archived messages by MAM is limited to 10000 by default. Beyond that, older archived messages are automatically pruned/deleted for internal storage. There is nowhere any reference in the present documentation about that. It also applies to its twin variable archive_item_count_cache Fortunately they can be set in your prosody.cfg.lua file. For reference: see source file mod_storage_internal.lua for these lines: local archive_item_limit = module:get_option_number("storage_archive_item_limit", 10000); local archive_item_count_cache = cache.new(module:get_option("storage_archive_item_limit_cache_size", 1000)); This is dangerous and highly unexpected behaviour, and should at least be included in the list on https://prosody.im/doc/storage.