[Dovecot] Transparent Migration from cyrus to dovecot

[Darren Pilgrim]

On 10/12/2013 3:43 AM, Noel Butler wrote:
> On 12/10/2013 19:22, Daniel Parthey wrote:
>
>> No mail will be lost, since it should remain in the remote MTA's mail
>> queue for a while in order to be retried and delivered later.
>>
>
> No guarantee there, some services are broken and do not retry, hotmail
> used to, and I've heard in some cases, still does, do this, some
> marketing system (ok, so thats no loss) do this - there reasoning is
> because of such high outbound queues, it would only delay first runs and
> upset their clients, again, no loss to me, but one persons spam can be
> anothers ham.
>
> It is after all why we have secondary MX's, on network, and if need be,
> off network.

Instead of deferring the message and returning a 4xx to the remote 
client, accept it normally and put it into a hold queue or defer the 
delivery transport.  After the switchover, requeue the message.