Mail is a read-only Counter32 giving the number of mail frames, for example Simple Mail Transfer Protocol (SMTP) traffic counted for the monitored network; per the MIB a value of 4294967294 means the count is unknown rather than genuinely zero. Because it is broken out on its own instead of folded into a generic IP total, an admin can watch this counter to see how much of the monitored network's traffic is outbound or relayed email traffic. If this Mail counter climbs sharply on the segment this probe watches while total traffic stays flat, that shift shows outbound or relayed email traffic is displacing whatever other protocol mix was there before.