"Message delayed" messages TO only one sender in the domain

Honored Experts.

I am the administrator of a small company running a SBS2011 server with 30 user connected to LAN with Win7+Off2010.

EVERY TIME one specific user is sending a mail to foreign mail addresses, she gets a message return saying that the message she tries to send is delayed.
If her colleague sends the same email to the same recipient it goes through without any problems.
 
This is the entire error message:
"Reporting-MTA: dns; SERV-SBS.INTERNALDOMAIN.local
Final-recipient: RFC822; NAME@DOMAIN.de
Action: delayed
Status: 5.4.0
X-Supplementary-Info: < #4.4.7 smtp;400 4.4.7 Message delayed>"

Normally I would say, that this was an indication of a problem in the receiving end, but then WHY is it only one of my user who is getting these messages?
OhmitAsked:
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systechadminConsultantCommented:
Please check if your domain have the proper PTR and SPF records. There are some domains who checks all these records before receiving the email. if anything is miscofigured, email gets delayed or sometimes expires.
OhmitAuthor Commented:
:-) how do I check this?
systechadminConsultantCommented:
Pull Out the DNS report of your domain.   http://www.dnsstuff.com/  this site has all kind of tools to check
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Muhammad BurhanManager I.T.Commented:
if your local server/Exchange is your SMTP so ask your ISP for adding PTR record against on your IP.
and if you are using some remotehost or ISPs SMTP so ask them for SPF.
OhmitAuthor Commented:
We are using our own DNS to send mail so our server acts as SMTP
Muhammad BurhanManager I.T.Commented:
Use Nslookup to check the DNS. Verify that the IP address is in IPv4 literal format. Verify that there is a valid DNS entry for the server or computer name in question. If you are relying on the FQDN in the Hosts file, ignore it, and update the entry in Exchange System Manager by using a valid IP address or correct name.
verify MX records validity from mxtoolsbox.com or any other resource.

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OhmitAuthor Commented:
Please notice that this occurs from one user only!
NOT the entire domain
Muhammad BurhanManager I.T.Commented:
Email her and make reply to that external mail then check the response.
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