After spamd handles the message:

Jan 30 14:05:22 prmx055 postfix/lmtp[31376]: DCBE81A581A1: to=, relay=prmx055.ourdomain.com[192.168.2.55]:7025, conn_use=5, delay=4530, delays=4529/0.58/0/0.06, dsn=4.0.0, status=deferred (host prmx055.ourdomain.com[192.168.2.55] said: 451 4.0.0 Temporary message delivery failure try again (in reply to end of DATA command))

Any thoughts as to what could cause the LMTP daemon to fall down on the job / mailbox daemon to not accept mail?

Thanks in advance...