[Dovecot] do not lose mail when dovecot is dead

[WJCarpenter]

> Eg. when a customer says: "Hey, I want my username to be "foo", and my 
> email to be "bar at domain.com". :)
>   

I encourage that on my servers.  It's just one more layer of defense 
against password guessing zombies; beats me if it makes any difference.  
I don't want to give them the userid for free just because they saw the 
email address on some spam list.