Using AOL as email provider the following error messages comes every two days:
2014-06-12 15:11:06 E-mail was NOT sent: 521 5.2.1 MUST COMPLETE IMAGE PUZZLE BEFORE SENDING. PLEASE GOTO http://challenge.aol.com/spam.html
SmartStruxure Lite MPM
This is AOL trying to stop people using it's email service to generate spam, you are now required to periodically enter a series of numbers or letters from a web page in addition to normal username and password authentication.