Brute Force Dictionary Attacks

I am getting several failed logon attempts, indicating someone is attempting to brute-force our system with a dictionary-style attack. What is the best way to prevent this? How do I prevent attempted logons from outside our domain?
EAS93Asked:
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eyeTSystemsCommented:
What services are visible to the internet on this machine?
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Mohammed KhawajaManager - Infrastructure:  Information TechnologyCommented:
Check event log and see which computer the failed logons are coming from.  You will be suprised how many times users create mapped drive which they logon with different accounts and configure the mapping to be permanent.  Before thinking it is brute force, analyze your logs first.
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WaronyaCommented:
Use only secure connections VPN/SSL for external access. Make sure the external access is only possible through a certificate.
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EAS93Author Commented:
mnkhawaja:
The failed logon attempts are coming from at least 3 outside ip addresses. 1 has been traced to New York, and another to Germany.
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eyeTSystemsCommented:
Logons cannot happen from external sources unless they have been granted access to attempt this logon.  
The failed logon attempts are coming from at least 3 outside ip addresses. 1 has been traced to New York, and another to Germany.
 This means that there is a service on this machine that has direct internet access.  What service (s) is/are they?
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EAS93Author Commented:
Outlook Web Access.
Please forgive me for the delay. I have been extremely busy.
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EAS93Author Commented:
Thank you. As stated in my previous comment, I've been really busy. I was hoping to get some additional feed back. I value Experts Exchange, and thank you for your help.
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