Error opening a string property... The error message was " could not be found."

Problem Description

When attempting to send queued messages with AspEmail's SendToQueue method and EmailAgent, the messages end up in the \Failed folder, and the following error is found in Event Viewer (Application section):

Error opening a string property from file C:\EmailQueue\xxxxxx.stg. The error message was " could not be found."

Solution

This error usually means the versions of AspEmail.dll and the EmailAgent service mismatch. Download the latest version of the AspEmail.exe setup from www.aspemail.com and re-install AspEmail and EmailAgent on both the machine calling Mail.SendToQueue and the one running EmailAgent.