Sendmail half-losing incoming mail. Ideas?

[Hack Reporter]

Every now and then, an item of mail arrives here, and lodges in
/usr/spool/mqueue as a dfAA* file, but without a corresponding qf...
file.
A recent example, sent from one of our direct uucp correspondents, starts:

	Mail failed (177400).  Letter returned to sender.
	From acorn.co.uk!andy Tue Jun 13 15:31:32 1989 remote from acorn
	Received: by acorn.co.uk (5.51/Am13)
		id AA05904; Tue, 13 Jun 89 15:31:18 BST

The letter was NOT, in fact, returned to the sender.

The uucp LOGFILE says:
	<some normal REQUESTED/COPY/OK lines>
	hhi acorn (6/13-16:53-12133) andy XQT (PATH=/bin:/usr/bin;export PATH;rmail igp at camcon.co.uk )
	hhi acorn (6/13-16:53-12133) ret (177400) from acorn!andy (MAIL FAIL)

We are running SunOS 3.4_EXPORT, with the standard uucp, sendmail and
a sendmail.cf based on the UK 1.4a.

Has anyone else had the same problems?  You may not know, since
/usr/spool/mqueue is root and 600. Get your local super-user to look in
it for unmatched df... files and see what gives.  Most of the items in
our directory were mailing-list stuff, so no-one missed it :-)

Please don't reply to comp.mail.sendmail, since we don't get that group.
My apologies if this problem has been dealt with there already.