Delivery can be counter-intuitive

We all know that receiving ISPs rate limit incoming email. With the volumes of mail that they’re currently dealing with they must do that in order to keep their servers from falling over.
A client was dealing with rate limits recently. These were not typical rate limits, in that the recipient ISP was 4xxing mail. Instead, the recipient ISP was not accepting any incoming connections. The client was having a bit of a difficult time understanding what was happening and why the problem wouldn’t be solved by increasing the rate at which they were trying to send to the ISP.
Imagine if you will, that at every ISP there is a reception desk that manages the incoming calls. The receptionist is under orders from the to limit the number of calls coming in. When the phone rings, the receptionist can do any of the following:
1) answer the phone and put the call through (250 message accepted)
2) answer the phone and put the caller on hold (connection hangs or delivery is slow)
3) answer the phone and tell the caller to call back later (4xx message deferred)
4) fail to answer the phone (no connection at all)
The delay the client was seeing was #4, in that they were attempting so many connections at once that the ISP was just not answering.
In this case, reducing the number of connections attempted worked. The “receptionist” was not so overwhelmed by the number of ringing lines that she was able to actually answer all the calls and put them right through.
While lowering the rate at which the client was attempting to delivery seems counter intuitive to getting improved delivery, because we understood the mechanism we could lower rate and get an increase in delivery.