The server sets a deferred recipient's next retry from its clock when the
attempt defers, in whole seconds. The test subtracted its own clock taken
when the loop next saw the message, after saving and reporting, so
whenever that lag crossed a second boundary the 2 s retry measured 1 s
and the test failed. Under load, after the other SMTP tests, that was
most runs.
It now measures from when the test started the attempt, which the
server's deferral can only follow, by under a second: each retry is its
interval or one more. Each position is still checked against its own
interval, so a wrong schedule still fails.
jcoffey-dev