Behaviour towards enemies:
Members of different sides cannot collaborate, communicate, or otherwise interact in any matter but as supreme, utter enemies. Fight or flee.
Communication between enemies:
You can taunt your foes, but you cannot facilitate their life in any way or kindly chat while you sip tea together. ANY kind of information exchange between enemies is strictly forbidden, unless it's misinformation (e.g. lying to the enemy to get them trapped is of course not illegal).
Duels are an honourable fight among foes who respect each other. The term 'honourable' does not suit Orcs and Trolls, therefore the only form of combat allowed between different sides is "fight your best or run like hell".
Linkless, idle, afk, and other OOC conditions:
It does not matter to the rules of MUME if the player of an enemy character has left the keyboard, lost connection et cetera. If you in any way help or protect an enemy (linkless or not), you are breaking one of the most important rules of MUME - this one.
See also: Rules PK