when I get an email from a friend of mine, she omits the To: line, how does she omit the TO: line?

Answer 1

She probably puts the "to" information in the field called "bcc". It acts the same as the "to" line, but does not list the "to" information in the e-mail. It is intended for businesses when they are "mass e-mailing" (or spamming) lots of people and do not want to make their entire mail list visible to everyone else. But it can also be used by individuals who want to hide the "to" information.

Answer 2

just enter the addresses in either the CC or BCC fields and not the TO field then send!