[Exim] Trouble with exim <-> TREND Micro viruswall

Hi,

I run exim 3.16. Some of my users send mail to a domain where they use TREND
Micro antivirus protection (www.antivirus.com). Some mails get bounced. Here
is a part of message log:

2001-05-28 15:02:05 SMTP error from remote mailer after RCPT
TO:<Radojka.Bronic@???>: host wall.pliva.hr [195.29.208.9]: 503 Need
MAIL before RCPT
2001-05-28 15:02:05 Radojka.Bronic@???: remote_smtp transport failed:
SMTP error from remote mailer after RCPT TO:<Radojka.Bronic@???>: host
wall.pliva.hr [195.29.208.9]: 503 Need MAIL before RCPT
*** Frozen (delivery error message)

What can I do?

TIA,

Hana