I think that actually, anything goes - excepting keeping to agreed rulesets and 'meta-cheating' (ie. reloading/hacking the save game/pw a la skyfish)
The thing is that in most pbem games treaties stick, and some people play rulesets that explicitly forbid treaty breaking. But unless you do have that agreed explicitly beforehand then it is allowed. By allowed I mean that (AFAIK, and I better be right!!) you would still be awarded the win (if it was adjudicated as a ladder game) - even if you were entirely dishonourable.
The fact that no-one ever seems to break treaties is just because they value their reputation over a win. If people really want cast-iron treaties they have to agree a ruleset that makes every treaty a 'game-breaker', ie. write into the treaty the fact that breaking it would forfeit the game.
all in imnsho, but as far as ladder games go I am right [
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