EventDeadLettersHealthCheck : EventDeadLetters contain events. This might indicate transient failure on event processing.

Hi,
I noticed logs being filled with

```
06:20:29.372 [WARN ] o.a.j.PeriodicalHealthChecks - DEGRADED: EventDeadLettersHealthCheck : 
EventDeadLetters contain events. This might indicate transient failure on event processing.
```

Is it something to be worried about?

There are a bunch of exceptions/errors (mostly regarding Date in headers, some handled and being 
logged as debug, some not logged as errors) - is the above line a result of those errors? For example:

```
08:57:28.150 [ERROR] o.a.j.e.d.EventDelivery$Retryer$BackoffRetryer - listener 
org.apache.james.mailbox.lucene.search.LuceneMessageSearchIndex exceeded maximum retry(8) to handle 
event org.apache.james.mailbox.events.MailboxEvents.Added
```

I assume above would result in the event being appended to the collection in EventDeadLetters (one 
of the implementations) and healthcheck just checks it, but shouldn't those be cleared at one point?

Btw. If I encounter error/exception while processing a message
-- 
Wojtek