From a qmail log, trying to send a 1204 byte mail.
2002-07-31 21:16:05.536651500 delivery 1500: deferral: Connected to 220.127.116.11 but sender was rejected./Remote host said: 452 Out of memory/
It's fair to run out of memory; but not on a 1KB mail. And not repeatedly.
$ host 18.104.22.168 Name: mc2.law16.hotmail.com