Email rejected when sent from ISP's SMTP

[seanatw]

Hi there,

We have been telling clients to use our server as both the incoming and outgoing server for their email, however we keep getting calls from them complaining about email messages they send to themselves or others on the same domain not working. (still using their ISP's SMTP)

I pulled this from mail.log

Code:

Dec 13 21:16:40 christchurchwebsites postfix/smtpd[30763]: connect from mta02.xtra.co.nz[210.54.141.253]
Dec 13 21:16:44 christchurchwebsites postfix/policyd-weight[26117]: weighted check:  NOT_IN_SBL_XBL_SPAMHAUS=-1.5 NOT_IN_SPAMCOP=-1.5 NOT_IN_BL_NJABL=-1.5 IN_IX_MANITU=4.35 CL_IP_EQ_HELO_IP=-2 (check from: .userdomain. - helo: .mta02.xtra. - helo-domain: .xtra.)  FROM/MX_MATCHES_NOT_HELO(DOMAIN)=2.087 CLIENT_NOT_MX/A_FROM_DOMAIN=5.85 CLIENT/24_NOT_MX/A_FROM_DOMAIN=5.85; <client=210.54.141.253> <helo=mta02.xtra.co.nz> <from=client@userdomain.co.nz> <to=client@userdomain.co.nz>; rate: 11.637
Dec 13 21:16:44 christchurchwebsites postfix/policyd-weight[26117]: decided action=550 Mail appeared to be SPAM or forged. Ask your Mail/DNS-Administrator to correct HELO and DNS MX settings or to get removed from DNSBLs; please relay via your ISP (userdomain.co.nz); <client=210.54.141.253> <helo=mta02.xtra.co.nz> <from=client@userdomain.co.nz> <to=client@userdomain.co.nz>; delay: 4s
Dec 13 21:16:44 christchurchwebsites postfix/smtpd[30763]: NOQUEUE: reject: RCPT from mta02.xtra.co.nz[210.54.141.253]: 550 5.7.1 <client@userdomain.co.nz>: Recipient address rejected: Mail appeared to be SPAM or forged. Ask your Mail/DNS-Administrator to correct HELO and DNS MX settings or to get removed from DNSBLs; please relay via your ISP (userdomain.co.nz); from=<client@userdomain.co.nz> to=<client@userdomain.co.nz> proto=ESMTP helo=<mta02.xtra.co.nz>
Dec 13 21:16:44 christchurchwebsites postfix/smtpd[30763]: disconnect from mta02.xtra.co.nz[210.54.141.253]

Why is this happening? and can I stop it from happening?

Thanks for your time!