The motivation for this is:
My test environment is not permitted to reach outbound port 25.
If I run an ad-hoc test without setting up an explicit sink,
I end up with messages that try to reach the public internet.
Since they are blocked at a firewall, each of the MX hosts in
the connection plan is subject to a 60s wait before trying the next
thing.
In addition, this can cause the shutdown to take longer while
we wait for the in-flight delivery attempts to complete.
Making a separate configuration option allows the local administrator
to decide how to split the time waiting for a connection from
the time waiting for the banner.
refs: https://github.com/KumoCorp/kumomta/issues/196
Wez Furlong
b003e49c9c