senmail delivery problem

[dEUS80]

hi!
i'm using sendmail 8.12.5 on red hat linux 8.0; in the
mail log i've found the following entries repeating:

Apr 22 12:01:59 linux sendmail[5724]: h3M81wxo005724:
timeout waiting for input from somehost during message
collect

Apr 22 12:01:59 linux sendmail[5724]: h3M81wxo005724:
from=<someone@somedomain>, size=0, class=0, nrcpts=1,
msgid=<200304220801.h3M81wxo005724@mymachine>,
proto=ESMTP, daemon=MTA, relay=samehost [someip]

Apr 22 12:01:59 linux sendmail[5724]: h3M81wxo005724:
to=<someone@mydomain>, delay=01:00:00, pri=30000,
stat=timeout waiting for input during message collect

and the message will not be delivered to it's
recipient; there are only a small number of ip's from
whom i can't receive e-mails, the rest of them
functioning normally;
why is happening this timeout waiting? the strange
thing is that message size=0 and msgid = <2003....
@mymachine instead of hismachine, hisdomain or
whatever;
it is possible that what the authors of netfilter
themselves call &quot;criminally brain-dead ISPs or
servers&quot; -# as i've read in this iptables tutorial:

http://iptables-tutorial.frozentux.net/iptables-tutorial.html

#- causes this timeout waiting?

can someone help me with this problem? please send me
your advice!
thank you in advanced!