Background

With Attach Plus configured to use Outlook, Send To->Mail Recipient operation results in the following dialog:

Warning: Attach Plus was unable to properly integration with your mail application (Microsoft Outlook).

Discussion

This error can appear if the user is using the 64 bit version of Outlook.   Attach Plus only supports integration with 32 bit email applications.

Resolution

Uninstall Outlook 64 bit, and install the 32 bit version.