If E-marketing is not enabled, the button will still be shown in E-mail & Documents and an error will be given if it is clicked.
This happens when the Custom_Sysparam for 'EnableEmarketing' is set to 'N'
The button for E-marketing will remain in E-mail & Documents until a metadata refresh is performed. If an IIS reset is ran then the button will come back.
If the button is clicked, the error: 'Functionality not available' will appear
Enable E-marketing using the following:
update Custom_SysParams set parm_value='Y' where Parm_Name='EnableEmarketing'
This case is currently with development and will be fixed in a future version of Sage CRM