Can no longer send email to three specific individuals

Suddenly My  client has a problem sending emails to about three different specific email addresses.  The outside person can send an email to my client, however my client cannot reply to that email

What I have done
1)      Checked the Outlook sent folder – The emails are there. So Outlook is not the problem.
2)      Sent the email from outlook web access with the same result, it bounced
3)      Sent the email from another employee of the same company and it bounced
4)      Sent an email from my personal gmail account from within the company and it went through
5)      Checked with Comcast to see if they are blocking any emails- they said they do not block emails and they did some checking and he was able to send to that problem email address from behind Comcast.
6)      Checked and turned off the email scanner on Symantec endpoint- the email still bounced
7)      Had the IT Tech from the recipient company turn off the spam filter and it still bounced.

Not sure what else to do. Any Ideas?
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dlennonConnect With a Mentor Author Commented:
the MXtoolbox was a valuable tool to use, Thanks.  Right now Comcast has escalted the case to the next level and hopefully they can help. It turns out anyone at the company we are sending too gets bounced
Dave BaldwinFixer of ProblemsCommented:
What are the bounce error messages?  How old is your Comcast modem?  Is it DOCSIS 3.0 capable?
dlennonAuthor Commented:
here is the error message

Reporting-MTA: dns; []

Received-From-MTA: dns; WS11 []

Arrival-Date: Thu, 06 Jun 2013 00:51:37 +0000

Final-recipient: rfc822; "recipient's email address"

Action: failed

Status: 5.1.1

Diagnostic-Code: smtp;  550 5.7.1 SPF uaa

I won'y be able to check which modem we have until tomorrow
Thanks for your help
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dlennonAuthor Commented:
This is the initial message. The previous post was the details of the message


This is an automatically generated Delivery Status Notification
Delivery to the followig recipients was aborted after 6 seconds
"email address"
Marshal HubsEmail ConsultantCommented:

Same discussed by MelvinTheGrate in this forum :

You can try to find solution here.
Dave BaldwinFixer of ProblemsCommented:
I don't know that it will show anything but MX Toolbox provides a way to check a lot of things about you email service.  I notice the message above mentions SPF which is sometimes used to determine whether an email is being sent from an acceptable source for the domain of the email address.  In simple terms, it checks to see if your email with a Gmail address is being sent from a Yahoo server in an attempt to impersonate you.
dlennonAuthor Commented:
Where is the SPF checking done?  From the message It appears like Comcast is doing the checking.  Is it their DNS server that does the SPF check?  If so, that is strange because Comcast told me they don't block any messages.
Dave BaldwinFixer of ProblemsCommented:
That is a little misleading.  Comcast and every other ISP in the land has spam filters.  I have one ISP that will block outgoing messages as spam every now and then.  Even when they let the original thru when I received it.

It is also possible that some one with Comcast Business class has their own email server.  I would think that the error message would have their server address instead of Comcast if that were the case.

DNS servers don't check anything, they just convert names into IP addresses.
Is it possible that all the staff at the same company are using the same cached autocomplete email address for the recipient?

Try a couple of things

If you go to one of the users.

Check their account settings to see if they have cached exchange mode on
If they do, turn it off and restart outlook

Then Start to type in the recipients name/email address

when it comes up as an auto complete option
Highlight it and delete it

Then manually type in the email address and send.
Do you still get an NDR?

If you do, also try entering the email address in the to Line
Now right click on the email address
Go to OUtlook Properties of the email address
Check to see what the email type is.
Is it Custom Type SMTP or Internet Type?

Also check that outlook decides best format
dlennonAuthor Commented:
When you say "Check their account settings to see if they have cached exchange mode onCheck their account settings to see if they have cached exchange mode on", is that in the exchange server or in outlook/  We are not running an exchange server.  It is running on comcast.  Where would I change the cache setting?
dlennonAuthor Commented:
I finally was able to get some help from Comcast.  As was shown by mxtoolbox site,  our name servers were still with AT&T.  I changed the name servers to comcast and I will know  in 24 hours if that will help.  I am a little worried that  the old name server addresses may hang around in cache somewhere. We will see
dlennonAuthor Commented:
changing the name servers did not help
dlennonAuthor Commented:
I actually allowed me to see that our name servers were still being hosted by AT&T.  Once we changed the name servers to Comcast, our email problem disappeared.  Since he pointerd me in the right direction, he can have the points
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