If there is ever an issue with email not being delivered, there are a number of factors which may cause email to queue on one of the relays. For example if a remote mailbox server is down (GMail or Hotmail for example) then emails will site on the first relay they hit until the mailbox server becomes available again.

If this is an extended period, after a while (around 4 hours) the relay will email the sender of a queued email to tell them that delivery is still pending.

These graphs give an indication of such an issue, you will normally see some emails queued and not being delivered, typically because of mis-addressed email, spamming, or faults with remote servers. If there is a major issue, expect to see a very steep and continuous upwards slope with a queue size in the 3-4 figure range.