smtp 550 5.1.8 error

We do not host our email.  We have an ISP that hosts it.  We send mail through another ISP.
But we do have an exchange server that is giving some of my users the smtp; 550 5.1.8 error sender IP must hae reverse dns (DNS PTR) for 158.123.238.21.
This does not happen to everyone.
If I as administrator send mail to same area everythng goes aok.  If I send via webmail under the user getting the error it goes aok.  Only if send via my exchange server our to our EMAIL ISP.

Help.

Jockbir@yahoo.com
jockbirAsked:
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december41991Connect With a Mentor Commented:
This problem can also happen if do not have proper exclusions for file level scanning. Exclude all Exchsrvr directories on all drives, and \\winnt\system32\inetsrv. Restart all AV services. This is suggested apart from setting reverse query as suggested by Raghu.

have fun

Dominic
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ExchgenConnect With a Mentor Commented:
I guess i should be in a good position to answer this...

First of all, try and get the outgoing IP of your exchange server / firewall. Get a "A" record created to it associating it with your domain (xyz.com).

You have indicated that you smarthost to your ISP, speaking of this you need to have the smarthost IP's reverse query should point / resolve to your domain ("A" record pointing to xyz.com).

Raghu
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