Thank you all for the answers!
Following Brad's answer, I looked into the mailman logs and found?
11045 mails sent! Yeah! Job was resumed (and completed) by Mailman.
Sendmail logs are a little different? it looks like about 50 of the
messages where lost and with Mark's post I assume they were in
"memory" at shutdown time. I can imagine that's a Sendmail
misconfiguration related to the queue checkpoints (off topic).
But I am very happy! 50 of 11045 is an acceptable "accident".