Like other sports, chess has its own code of conduct and its own idea of what constitutes sporting and unsporting behaviour. Some types of bad behaviour are expressely forbidden. For example you are not allowed to move a piece with one hand and press the clock with the other (Article 6.2.3 of the FIDE Laws of Chess), because this is an artificial attempt to shorten one’s own clock time and lengthen the opponent’s clock time. It is also forbidden to distract or annoy the opponent in any way during the game (Article 11.5).
One might argue that chess etiquette simply reflects the more general obligation to treat other human beings with respect and consideration. So, although not required by the rules of the game, it is the done thing to shake your opponent’s hand at the start and end of the game, and to say something polite like “Good luck” at the start and “Well played” or “Thanks for the game” at the end.
Not everyone behaves correctly, or course. Some of my opponents have remarked at the end of the game, “You were lucky to win that,” or “This was not one of your better games,” or “Tonight you did not play to your grade” (say that last one in a Russian accent and you’ll know who I mean).
Similar considerations apply in online correspondence chess, although notions of etiquette may seem more remote when you cannot see or hear your opponent. “Distracting your opponent” doesn’t have the same force when you have days rather than minutes to make a move, and you don’t have to sit at the computer or stay in the same room the whole time.
Even so, I was told at the start of my correspondence career that it is good practice at the start of a game to send a greeting or best wishes to your opponent. And I can see the point of ending the game on a gracious note as well. I regard this sort of thing as a virtual handshake.
What happens if your opponent does something during the game to which you object, but which is not serious enough to engage the ICCF code of conduct? For example if your opponent turns down a draw offer in a position which is objectively drawn, simply because he is higher-rated. This is, in my view, a sign of disrespect. I am often tempted to make some cutting remark in a message with my next move. Maybe “You must be either arrogant or stupid.” Or “I can recommend a basic guide to rook and pawn endings.” Or just “Seriously?”
However, I have so far managed to restrain myself. I know that it would be unacceptable to say these things over the board and I am trying to apply the same standards online. It’s better to remain silent and just keep on making moves.
Even so, I don’t think players should be able to get away with it, so I do allow myself a little licence. In respect of abortive draw offers, when the game comes to an end I don’t make the usual pleasant remark. Instead I say nothing at all. This is the virtual equivalent of refusing to shake hands. It doesn’t have the same effect if your opponent cannot see you ignoring his outstretched hand, but I feel that I am making a point, at least virtually.
Another, more concrete, way of expressing disapproval is if your opponent unreasonably declines your draw offer and the position later simplifies to one with 7 or fewer pieces on the board and so is amenable to a tablebase claim. In these situations I will (obviously) check that the tablebase verdict is going to be a draw, then I will simply claim the draw without making a further offer. Similarly if my opponent plays on in a losing position and we reduce to 7 pieces, I will simply claim a tablebase win.
By extension, if a position is drawn, and there are 7 or fewer pieces on the board, and neither side has made a draw offer, I regard it as bad practice to claim the tablebase draw without first offering the draw to your opponent.