Here are a few suggestions for the conduct of a CC game. Some are fairly obvious. A few are personal and may not find universal agreement. Use this list as a starting point and form your own conclusions about proper CC etiquette.
Respect your opponent
Chatting can be interesting but avoid subjects such as politics and religion.
Respond to correspondence
When an opponent makes a comment feel free to reply, but if it is obvious your opponent does not want to chat, then don’t insist on it.
Play strictly by the rules
It is not bad sportsmanship to expect your opponents to play by the rules, and you should do the same without question. If your opponent oversteps the time limit, do not hesitate to claim the win. Likewise, if you violate the time control and your opponent claims the win, loss of the game is your fault, not his.
If site rules do not allow for engine use, then don’t use one. If engine use is allowed, it is not cheating to use one. If engine use is not allowed and you suspect your opponent is using one, don’t rail at him; use the proper reporting procedure and let the site admins handle the problem.
Don’t whine that engine users are not learning anything because people who use engines illegally are not trying to learn anything; they are trying to win the game by any means available.
If you are using correspondence games to hone your OTB skills and choose not to shift pieces when analyzing, use opening books or endgame manuals, etc. that is your choice. If your opponent is using books, that is his choice. Books have been allowed in correspondence chess from the beginning and no correspondence organization has rules against their use.
Taking advantage of mistakes
There is absolutely nothing improper about taking advantage of an opponent's mistake. I have played on a site that allowed takebacks and if an opponent asked, I usually gave them one because I didn’t care about the results. On most sites this is a non-issue because takebacks are not possible.
Send a final message
When the game is over, regardless of the result, send a final note. There’s nothing wrong with sending a message saying well-played, thanks for the game, good luck in the rest of the tournament, or on some sites, sending a handshake, etc.
Don’t comment on your opponent’s mistakes
If you make a terrible move don’t crow about how it was a mouse slip or you were analyzing the wrong position and confirmed your move without looking at the board. Yes, it happens; I’ve done it.
If you opponent makes a howler, don’t say anything. However, I don’t see anything wrong with telling an opponent when he made a great move.
Do not ask your opponent to resign
It is always annoying when an opponent plays on in a lost position especially if they drag things out by waiting until the last minute to move but it is never appropriate to ask an opponent to resign. As Campbell put it, “In this case you should let your chess moves do your talking.”
Remember, especially if you are playing a low rated opponent, he may not realize his position is hopelessly lost.
Playing on in a bad position
If you feel you can learn something, still have a defensive resource, the position is complex or can be made complex or you are unsure that your opponent really has a won game, then by all means play on! But, if you are totally busted, man up and resign.
Avoid analyzing the current game
Vague general comments are OK, but avoid listing possible variations or giving detailed evaluations.
The worse thing a correspondence player can do is to disappear without trace. If you choose to quit for any reason you must notify your opponents and tournament secretaries. Of course there are exceptions to this rule where it will not be possible to notify everybody that you cannot continue, but they are exceptional.
Remember the "Golden Rule"
As Campbell put it, “In your correspondence, treat your opponent as you would like to be treated. We are all friends sharing this wonderful experience that is called correspondence chess.”