> If I chose "defer" verdict I obtain:
> 450, '4.7.1 <[email protected]>: Recipient address rejected: Policy rejection; 
> Message count quota exceeded

> In no case the mail is queued, but is removed.

450 is a temporary failure code - the sending client should queue the message 
and retry later. In the case of an MTA (Mail Transfer Agent) this should "just 
happen", but in the case of an MUA (Mail User Agent, aka mail client) this 
usually just results in an error.

For this reason, throttling mail sent my MUAs to very tight limits is generally 
not a good idea. Depending on the MUA it may result in various combinations of 
error messages/dialogs and/or require manual intervention to restart sending.

Another reason to avoid "tight" limits such as these is that even though an MTA 
will queue and retry, if there are more then 3 recipients, each delivery 
attempt will fail. It depends on the configuration of both server and clients, 
the message may go through to 1, 2, or 3 recipients at a time until all are 
done - or it may "get stuck" and fail to send at all.

So I'd suggest a larger number of messages over a larger time period. 2 
messages ever 10 seconds is the same average rate as 12 messages every minute, 
or 120 messages every 10 minutes, or 720 messages/hour. The only difference is 
the "burst" capability.
To understand that, consider the quota as being like a leaky bucket. When no 
mail has been sent for a while, the bucket will be empty, each time a message 
is sent, water is added to the bucket and the level rises, and when the level 
is above the cut-off point then further messages are rejected. Over time the 
water leaks out and the level goes down, then further messages are accepted.
If the bucket is large, and the cut-off level high, a lot of messages can be 
sent initially - but once that cut-off level is reached, messages can only be 
sent at the rate the water leaks out. That rate is the same for all the quotas 
listed above - roughly one message every 5 seconds. The difference is the burst 
needed to hit the quota in the first place : 3, 13, 121, 721 messages* 
respectively.
But the overall rate is the same in all cases - with the higher burst ability, 
there also comes a longer period to completely reset it. Eg, if you sent 120 
messages in one go, you'd need to wait a full hour before you could do the same 
again.

* The cutoff is normally the quota level +1. Messages are only rejected if the 
counter is above the limit, and the "bucket" is draining from the instant the 
first recipient is logged. So after sending a message to 2 recipients, the 
quota will be 1.9<something>, it needs 3 recipients to put the quota counter 
over 2. In practice, the 731 is probably conservative since by the time the 
731st message is sent, enough time will have passed (and hence the bucket 
drained) to allow another message or 2 through.


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