4 yo mare just became agressive all of a sudden in the coral.

If you're new to horses & not knowledgeable, I would strongly suggest finding yourself a good trainer/instructor to teach you. Regarding 'manners' & what is/isn't allowed, I think you need to think of the 'what ifs' & safety precautions of what you might allow or ask for and be utterly consistent about your rules. On that note, I agree with Ricci that regardless of what an experienced person may/may not allow or invite(not 'let'), it's safest for a beginner to avoid anything that can easily become a safety problem.

With that particular impolite behavior my response is to start rubbing and scratching a little harder than what the horse actually enjoys. That usually gets them to change their mind about rubbing on me.