I have one customers that stop sending email (delivery) to their intended recipients, but not all. For example, they can successfully send email to @gmail.com, @msn.com, and other company email addresses, but not to dsa.mil, cisa.mil and other companies that use public spam databases e.g. abuse.net, http://www.spamhaus.org/, messagelabs.com (e-mail service), etc.

My first impression is their email server public IP address is listed in one of these public spam database. So I head out to visit http://www.mxtoolbox.com/ website and use their Blacklists tool. This is a army swiss box tool for any IT Consultant out there, all you have to do is check your client e-mail server public IP address and wait for the result.

If you're in their website, try the Email Header Analyzer tool as well. You will be surprise how easy to troubleshoot an email flow.

I thought this is a useful tool for everyone in information technology.