On Sep 4, 2018, at 3:33 AM, Jan-Pieter Cornet johnpc@xs4all.net wrote:
Our current software no longer has that limitation, and we now close the connection 30 seconds after delivery (unless new mail arrives that can reuse the connection).
One off-topic comment on this subject. Postfix also has support for connection caching, but uses the feature much more carefully (on demand):
1. Connection caching to a destination is only enabled when the queue has more messages to that destination than the destination concurrency limit. In other words, only when there are messages waiting for a delivery slot queued behind the current message, and so the cached connection is likely to get used.
2. Cached connections are closed after 2s of idle time, only sustained traffic keeps cached connections open.
3. Cached connections are closed after ~300s of use. This amortizes connection setup latency when some MX hosts are slow. Sadly some receiving sites limit the number of messages per connection (rather than connection duration). That's unfortunate, re-use limits by message count don't overcome slow MX connection "attractors".
See http://www.postfix.org/CONNECTION_CACHE_README.html#safety
These ensure that connection caching is never seen as "aggressive" by receiving systems.