Cron's emails

"SKIPPED: postgresql-52896-" Can anyone tell me where this message is coming from and what it means? It appears every night at 20:05. It's generated in the "-*/15 * * * * root test -x /usr/lib/cron/run-crons && /usr/lib/cron/run-crons >/dev/null 2>&1" which is automatically installed in crontab but I can't trace though to find its source. After that line in crontab, I have MAILTO="" which stops the jobs I've added from telling me they've fired off but I'd like to keep the 'standard installation' as-is. TIA John