As per Microsoft: "Associations are paths that are opened to other systems. Each association is contained within a connection and is used to transfer messages to a system. You may have multiple associations in each connection". See MSEX2KDB for additional information about this event.
Exchange Server bridgeheads configured with a large number of site connectors may experience occurrences of this event. It indicates that the message transfer agent (MTA) has reached its limit for concurrent LAN-based MTA connections. Currently, this limit is approximately 240 concurrent associations for each MTA. See ME243402 for further details.
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