Weeeeeeell that depends. I work with a lot of lawyers and i know the unwritten rules of the game pretty well if you want the basics.

Email is good. It gives you a forum to show biz as usual and utterly swamp anyone with boring ass details as long as you dont have idiots that actually TALK ABOUT SHIT that might be a legal liability in email. Email is generally covered by document retention rules and you know you will possibly have to hand it over one day, so chat away as long as it isnt anything that anyone is likely to sue you over.

Then there is the occassional "is this entirely legal?" subject. Nothing too bad. Things like "hmmmm. The contract we signed is kind of vague here and im not entirely sure we are fullfilling it." For this, even if the person that NEEDS to know is say the CTO, you don't send it to the CTO. Rather, you play jeapoardy and put your statement in the form of a question and send it to your lawyer with the CTO in the CC line. "What are the possible interpretations of this line of the contract?". You mark this priviliged because you are seeking legal advice and these can be easily indexed and removed during subpoena as attorney client privilege and are not admissible in court.

Now lets say we need to have a little chat with the CTO, but really, we are on the hook for a few grand at most we think. Well... IM isnt a bad option! Normally its pretty sheltered from document retention as opposed to email.

Weeeell, we find ourselves in the situation where its bigger than we thought. You pick up the phone with lawyer and CTI. Nothing in writing and privileged.

Crap. This is big. Time for face to face only. No phones.

This is existential. Turns out we may be on the hook for a billion dollars since that clause we failed breached a major patent license of our fortune 500 partner. We no longer use offices. We only speak of it while taking a casual stroll at lunchtime by the river.

/if you ever get to that last one... Good luck.