What does “delivered” actually mean in ESP reports?

When your ESP reports a message as delivered, it means the receiving mail server responded with a 250 OK during the SMTP transaction. The server accepted responsibility for the message.

This tells you nothing about placement. The message could be in the inbox, spam folder, quarantine, or even silently discarded by post-acceptance filtering.

Delivered simply means accepted. It is not a guarantee of visibility, engagement opportunity, or inbox presence.

Your ship was allowed into the harbor and logged at the port authority. But you have no idea which pier you were assigned until someone goes looking.