When is it most appropriate to quote from an e-mail message in a response? when you want to simply write to the sender without starting a new e-mail when you want to include specific confidential or embarrassing information when the quote makes the response e-mail too long for the recipient to read when the quote is part of a series of e-mails written by more than one person

When the quote is part of a series of e-mails written by more than one person.

Explanation:

This is one occassion in which it would be appropriate to quote from an e-mail message in a response. In this example, several e-mails are being exchanged. Because of this, it would be easy for someone to get confused as to the order or the origin of certain pieces of information. This would be especially true if the emails are written by more than one person. By quoting from the e-mail, the sender ensures that tracking information within these emails becomes easier.

so much for the extra points :