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Mass Mailing with Entourage and Outlook Web Access
There is a known issue with Microsoft Entourage for Macintosh and Outlook Web Access (OWA) light edition (light edition is default when non Microsoft Internet Explorer web browsers are used), which under certain conditions, causes recipients in the Bcc: field to not be sent. It seems the issue has been fixed in version 12.1.7 of Microsoft Entourage.
To check your version, open Entourage and select "About Entourage" in the Entourage pull down menu. If you are not running this version, you can upgrade by selecting "Check for updates" in the Help menu of Entourage.
The issue is when sending to a group of recipients and the recipient field (to: cc: or bcc:) is more than 1000 characters the field is truncated at the 1000 character mark.
There is no current fix for this in OWA light but a new version of this will be released from Microsoft in the future with all of the features of the full version of OWA including multi-platform and multi-browser support. A workaround is to send large groups of mail in batches of less than 1000 characters or use a mail client like Entourage 12.1.7 from Macintosh or Outlook 2003 or Outlook 2007 for Windows.