Office XP Outlook occasionally sends tnef/rich format files instead of PLAIN TEXT, This problem is apparent only to non-Exchange users of Email who receive these messages. The problem is caused by an Outlook cache setting. If upon your first send to an email address Outlook decided via some automated wisdom that the default Plain Text should not be used, a permanent cached preference is stored that cannot be changed without flushing the cache.

To determine if this your problem:

• Type the offending email address into the TO line of a blank email message
• Hit the "Check Address" button (looks like a person and a check mark); this should cause the email address to resolve and you will see the address become underlined
• Place your mouse over the email address and double click
• A separate window will open labeled email properties
• There is an "Internet Format" setting in a drop-down list; choices are
  • Let Outlook decide the best sending format
  • Send Plain Text Only
  • Send Using Outlook Rich Text Format
  You will find that while you can select a different format for this particular message, you cannot save your changed preference (IE you cannot force the cache to save what it has decided to store for this setting). If Outlook is using the wrong behavior, you MUST force the cache to clear.

SOLUTION:

To clean the cache:

• The command to clear the cache is manual
  outlook.exe /cleanfreebusy (you might need to use full path to outlook.exe)
• Please note that ALL CACHED SETTINGS ARE DELETED