Intermittently seen on Extranet, this has been confirmed to be a bug with the mail queue on all clustered installations.

The queue implementation uses a clustered cache as its storage mechanism, however the cache is not configured correctly. The configuration is just a normal cache, with a one-hour expiry and maximum of 1000 items.

In some situations (mail server outage, very high volume of email), losing an email from this buffer prevents the buffer from sending any subsequent emails.