'no relaying allowed' email errors on OS X - mDNSResponder

Today I'm receiving mail delivery failures when I send email to some domains from Outlook on my Mac. It was mDNSResponder's fault.

Email:

Reporting-MTA: dns; rcdn-iport-6.cisco.com

Final-Recipient: rfc822;recipient@domain.com
Action: failed
Status: 5.0.0 (permanent failure)
Remote-MTA: dns; [123.12.34.56]
Diagnostic-Code: smtp; 5.1.0 - Unknown address error 554-'No relaying allowed - psmtp' (delivery attempts: 0)

And the solution - restart mDNSResponder from the command line (Terminal):

sudo killall mDNSResponder