Exporting emails via drag and drop: 10% of exported emails suddenly have zero kByte size

Nearly 10% of my emails show a strange behaviour:

When I select and export an email message via drag and drop onto the Mac OS desktop and look at it (Cmd-i), it says that this email (.eml) has zero kByte file size. Zero kB! But within Outlook, it has content, some even have attachments, some emails are really big, others are one line of text only. But in any case, ZERO KBYTE can't really be! When I put this Zero-kB EML back to Outlook it then is empty. Of course, because on the desktop the file size was already null.

I tried to find similarities between these zero-emails, but nothing seems to be the source:

- different sizes

- different senders

- different history (some are forwarded, some come directly to me)

- some have attachments, some do not

Rebuilding the database didn't help.

When I Ctrl-Click one of these Zero-kByte emails within Outlook, and I want to choose "Show source" (German original text "Quelle anzeigen"), nothing happens. Only these zero-emails can't show their "Outlook_MessageSource".

My system:

MacPro 2.8 GHz QuadCore Intel Xeon, 16GB RAM

Mac OS X 10.9.1

MS Outlook 2011 (Vs. 14.3.9)

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Mathias