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Hiding the list of the recipients' addresses in an email message
When you send an email to several people, the "To:" or "For:" field should preferably feature a single address (your secondary address, for example), and in the "BCC:" or "ICC:" field, you may include your various correspondents' addresses.
These "BCC" and "ICC" fields ("blind carbon copy" and "invisible carbon copy" respectively) will make your recipients believe that they are the only person to receive the email, or at least hide the addresses of other people, which avoids the propagation of spam in general and provides a welcomed confidentiality.
The "ICC" and "BCC" fields should not be confused with the "CC" field which does send copies to your recipients, not hiding their respective addresses however!