Outbound volume is the amount of data or number of messages a system sends to other systems over a period of time. Email operators may measure it by messages attempted, accepted by receiving servers, or successfully delivered; those measures are not interchangeable.
Monitoring outbound volume helps defenders spot compromised accounts, spam, phishing campaigns, data theft, or misconfigured services. A sudden increase in outgoing email can trigger investigation or rate limits, but volume alone does not prove who sent the messages or whether recipients received or acted on them. Responders should compare activity with normal patterns and preserve mail-flow logs, timestamps, and related connection records.