Hi Lex,

From the description, you got the error message "message interface has returned an unknown error." when exporting Outlook data to a PST file, correct?

Based on my research, this issue may happen if one of the following two conditions is true:

• There is some internal problem with Microsoft Outlook messaging interface.
• There is corruption in PST file.

If it's due to Microsoft Outlook messaging interface, this issue should be fixed after restarting Outlook. If it is caused by Outlook PST file corruption, we can try using the Inbox Repair Tool (Scanpst.exe) to scan and repair your PST file. By default, we can find the Scanpst.exe tool under:

For Outlook 2007 on a 32-bit version of Windows:

<disk drive>:\Program Files\Microsoft Office\Office12

For Outlook 2007 on a 64-bit version of Windows:

<disk drive>:\Program Files(x86)\Microsoft Office\Office12

For more information, see: https://support.microsoft.com/kb/272227

In addition, if your PST file is too large and exceed the maximum size limit, it may also cause PST file corruption. In Outlook 2007, the PST file size limit is 20 GB. Please make sure your PST file doesn't exceed the limit.

Regards,

Steve Fan
TechNet Community Support