SYMPTOMS

MailEnable's web mail returns the following error when any user attempts to login to read their mail.
"A system library error occurred when calling ADO. ErrorNo: 3704; Description: Unknown runtime error "

CAUSE

Please note that this issue is a generic system error and is not related to the MailEnable product suite. The issue is possibly caused by third party software corrupting the system MDAC/ADO configuration. Other applications using ADO/MDAC in the same way as MailEnable are likely to also fail unless this issue is resolved.

The reason this error is returned is most likely a conflict or corruption in the system ADO libraries that are installed on the server.Some versions of Coldfusion (4.5 and perhaps others) have known problems with corrupting the Microsoft MDAC Registry keys, as described in Microsoft's Knowledge Base here: 
http://support.microsoft.com/default.aspx?scid=kb;EN-US;259569

RESOLUTION

Microsoft Knowledge Base articles relating to this error usually recommend a reinstall of the operating system. Also, review the Macromedia site for additional directions in repairing the ADO environment/system libraries.

Whilst the advice supplied by the respective vendors should be considered, the following procedure should be considered.

Note: Ensure that appropriate backup and rollback steps are taken - given that there is no vendor or MailEnable endorsement for the following procedure:

1. Use the ADO Component Checker to inspect the ADO configuration. The utility can be found at: http://msdn.microsoft.com/data/downloads/tools/default.aspx
2. Re-install the current release of Microsoft's ADO Libraries from their web site:
3. An unsupported (and last resort) tool for repairing the MDAC/ADO system configuration can be found athttp://www.betav.com/Files/Content/tools/tools.htm. The tool is known as "mdacrt.exe"