My daughter, Aoife, was asking what does RE: mean in an email. Well….
“Re” is a Latin word meaning “about”, as is ”
“Re” has probably been used in correspondence since before there was correspondence in English.
Now, most people think of “RE” as an abbreviation for “REgarding”.
Davy's Blog 
I always thought in emails in means ‘reply’
re is an abbreviation of ‘REference to’
This is abbrviate back in the days to save ‘memory’ on the pcs.
No, it really is from latin. More literally, re stands for “by the thing”. From the ablative of res (“thing” or “circumstance”). It is a common misconception that the “Re:” in correspondence is an abbreviation for regarding or reply; this is not the case. The use of Latin “re”, in the sense of “about, concerning”, is English usage.
And yes, it’s used in legal texts too.
It is true. “RE:” is short for the Latin phrase, “Per Res” which means “by the thing.”
It mean a side note or an after thought, but has come to mean “in reply/response to.” or “regards/concerning.”
It all comes down to following up on something someone else has written or said.
Best to you all!
I was just thinking the simple route and said “regarding”