Office 2007 - "The messaging interface has returned an unknown error...

We run Microsoft Xp at my office and we upgraded to Office 2007 about six months ago. It worked flawlessly until about 2 months ago, when I started receiving an error upon attempting to send an email message that contains a Microsoft Office file -- all other files work okay.

The error that pops up says:

"The messaging interface has returned an unknown error. If the problem persists, restart Outlook".

The problem seems limited to two situtaions:

1) Sending an email from within an Office program
2) Right clicking a file and selecting "send to--> mail recipient"

Oddly enough, it doesn't seem to be a problem if I'm sending new files (for example *.xlsx), only with .xls files. Same goes for .doc and .docx etc

I've deleted my Outlook profile, removed my .ost files, clicked on the "fixmapi.exe" file, reinstalled Office...and the problem persists.

The only programs I've installed since upgrading to Office 07 are Visual Basic Express 2010 and Adobe Reader (which has caused other issues e.g. breaking the PDF previewer in Outlook). At one point I had removed all Adobe software and registry entries -- still, nothing.

Any ideas?

I am not sure about that particular error, but since you brought up that you can send newer files...
With Office 2007, Microsoft changed quite a few things for security, one of the main one is how they handle macros. For starters, excel and word now have 2 file formats/extentions. One allows macros and the other doesnt. The macros security settings within the applications are based off the file extension, so if you open a macro file you will get a security warning (if it enabled), but if you open one of the older files you will not get a warning, even if it contains macros. So for this reason Outlook has been set by default to block the older file versions. If you are on a corporate network, they may have their systems configured even more strictly.

You should be able to find more info if you look in here:
http://technet.microsoft.com/en-us/l...ffice.12).aspx

Or google it...