[Exim] Problems sending mail to Group Wise ( 503 )

Hello,
I'm having problems interoperating with Group Wise GWIA ( 6.0.2?),
it sometimes says :
SMTP error from remote mailer after DATA: host host.na.me
[127.0.0.1]: 503 Bad command sequence

according to exim source all 5xx error messages mean that message is
undeliverable permanently, but when I saved one of trouble-causing
messages and tried to send it at later time it went through OK.
What is more, most of the problems are with messages with multiple
receipients, and few of first go ok, rest fail with 503...
I tried modifying exim so it won't mark 5xx as permanent failure, and
now when there are problems it freezes offending messages, and when I
unfreeze them they go through ok.

What should I do?

regards, Eyck.