550 Invalid RDNS entry

I have a client that says he is getting a bounceback when sending a message to our domain

mail.summit-inc.net rejected your message to the following email addresses:
alfred@st-inc.net (alfred@st-inc.net)
mail.summit-inc.net gave this error:
Invalid RDNS entry for

I went to DNSSTUFF.COM and looked up the rdns for our domain and it looked correct.      PTR      IN      3600      41ms      co*****.com.

Is there  another reason why the clients message would get rejected?  I don't really understand the whole rdns thing in the first place so I guess it could very well be incorrect!
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It is not clear to me, but i can say you this. sender's server ip should have a correct PTR (RDNS) entry setup.
If alfred@sc-inc.net is the sender mail.summit-inc.net should have a proper PTR (RDNS) Eentry.
If co****.com is sender then, mail.co****.com should have proper PTR RDNS entry setup.
Thank you
If someone is getting rejected by your domain, it should be their PTR record which needs to be corrected.

Depending on email filtering settings, a receiving server can look at the IP of the incoming connection and check whether there is a PTR associated with the IP.  Further it could also check to make sure the name returned by the PTR record has an A record that references back to the same IP.  This is a spam prevention feature as most dynamically assigned IPs won't have a PTR record.
Amit KumarCommented:
Can you provide your domain name to check PTR records.

or you can do it via mxtoolbox.com

PTR's domain name and MX name should be the same in this case.
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you need to check sender's PTR (RDNS) record in order to resolve this. this happens sender's host name does not have a proper RDNS entry setup. when you receive a mail from external domain your mail server checks from where it comes & if it does not match RDNS of sender's host name it will be rejected from your mail server. but the sender you specify here will not possible to send mail to almost all mail servers in the world if it has issues with RDNS entry. you can check this with them.

NytroZAuthor Commented:
The domain name is context4.com.  We dont' have any RDNS issues with anyone else but this single domain we are sending to.
NytroZAuthor Commented:
This is the record that is coming back.  Why is this incorrect?      PTR      IN      3600      41ms      context4.com.
You need to be clear.  First you said there was a problem with receiving email from a client on another domain, then you said there was a problem sending to another domain.  Which is it?
NytroZAuthor Commented:
I apologize....our employee is getting an error sending to the client.  The error states that or rdns is incorrect

Context 4.com cannot send to summit.com
I don't believe I've ever seen multiple PTR records for a single IP before, so I couldn't say if that could cause any issues.
I think the domain you're sending to is doing a forward-confirmed reverse DNS check.  If the IP resolves to "context4.com", "context4.com" doesn't resolve back to the same IP.  Instead your PTR record should point to "mail.context4.com", which I see you have an A record for which points to the same IP.

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Amit KumarCommented:
your PTR is as below:

PTR      context4healthcare.com      60 min
PTR      context4.com      60 min

and your MX record is as below mentioned:

mail.context4.com      30 min      Blacklist Check      SMTP Test

create one PTR for mail.context4.com with IP
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