We just released the latest update to Email Marketing; 4.61.
This update contains a number of bug fixes and enhancements for the sending engine (primarily) and other parts of Email Marketing. A complete list of the changes can be found here.
Some users experienced issues with the 4.6x sending engine. These issues have been resolved and in the process of resolving those issues we further optimized the sending engine. Resulting in a faster and more stable sending engine for all the platforms tested.
Should you have any questions and/or feedback just let us know.