Answer
PROBLEM
Email fax requests fail with the following error: '451 4.4.0 primary target IP address responded with 421 4.2.1 unable to connect'
ENVIRONMENT
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GFI FaxMaker installed on a separate machine
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all supported environments
SOLUTION
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Open the Services console by clicking Start > Run > and typing 'services.msc'
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Locate the service Simple Mail Transfer Protocol (SMTP)
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Set the service to Automatic and start it
CAUSE
The Simple Mail Transfer Protocol service is not running and set to start manually