"Re:" as "Regarding" or "In Re" is Redundant in Subject Lines
Contrary to other contexts, "Re:" does not mean "Regarding:" when used in email subjects. The subject line of an email message already contains "Subject:" to indicate the line's purpose.
(Strictly speaking, "Re" is an abbreviated form of the Latin "in re", which, "re" being the ablative case of "res" (thing), means "concerning" or "in the matter of".)
"Re:" is for Replies
"Re:" should only be used in replies.
If you put "Re:" in front of the subject of a message that is not a reply, the recipient will be thoroughly confused, thinking they received a reply to a message they did not write with a subject they do not know.