MailHog doesn't appear to impose an upper limit on the amount of memory it could allocate to the message queue. Assuming that people aren't going to bother deleting email most of the time, won't this become a time bomb on a long-running server?

A sensible default limit should be imposed. Question is what to do once that limit is reached - do you auto-delete old messages or return a non-delivery error?

Relates to #74