I'm having a similar, but slightly different problem. 'mailq' reports that postfix has several messages queued for local delivery. Backups were not running during the time that these messages were received. Most messages are received and delivered without a problem.
Code:
4490129506B8 883537 Mon Dec 12 19:18:45 johnxx@inreach.com
(host z1.sfo2.xxxx.com[209.xx.xx.xx] said: 451 4.0.0 Temporary message delivery failure try again (in reply to end of DATA command))
dan@xxxxx.com