Figuring out postfix errors

I recently faced a problem with email from one of our unix nodes. We have a system (a set of bash scripts) which sends out email on completion of a test suite to various mailing lists. One fine night, it just broke - stopped sending email to anyone. On some investigation, on the machine from where emails were to be sent, found this:

mc1 04:52:50 /var/spool/postfix/defer $ pwd
/var/spool/postfix/defer
mc1 04:52:51 /var/spool/postfix/defer $ ls
1 3 4 5 6 7 8 9 A B C D E F
mc1 04:53:29 /var/spool/postfix/defer $ cd 1
mc1 04:53:32 /var/spool/postfix/defer/1 $ ls
14BA8540BB 164E8540C5 191C9540E0 1C77B540C6
mc1 04:53:34 /var/spool/postfix/defer/1 $ cat 14BA8540BB | head

: host relayhost.domain.com[ip appr] said: 452 Insufficient system storage (in reply to RCPT TO command)
recipient=username@domain.com
offset=423
status=4.0.0
action=delayed
reason=host relayhost.domain.com[ip appr] said: 452 Insufficient system storage (in reply to RCPT TO command)

I have of course replaced real machine names and IP addresses with dummies. There were also some "connection refused" messages in the files here, which were probably an offshoot of the root problem.

Haven't found what's to be done yet - will update this when done!