You might try the patch mentioned in a related thread (“OSX 10.6.5 , Terminal.app, iTerm.app pbpaste, pbcopy do not work under tmux”) from the tmux-users mailing list (actually hosted at sf.net, but I abhor their archive interface).
Note: the patch was, at one time, applied to the MacPorts port, but it was later reverted when it was suspected to be causing other errors. However, those errors might have actually been caused by liking against the wrong version of libevent (and it seems that recent versions of tmux will work with either version of libevent).
Since helgeg brought it up, I’ll mention my notes on my attempt to use the above referenced patch.
Summary: I found the patch unreliable. It allowed pasteboard access, but it triggered many warning messages from my installation of libevent version 2 (maybe version 1 is less affected). It also prevents the tmux server from doing its normal fork(2) (via daemon(3)), which is a significant change to how the server process was designed to work.
I ended up publishing a program that works around the problem without patching tmux. People that have had problems using the pasteboard under tmux (and those for which the above patch does not work satisfactorily) might want to give my workaround a try.
tmux new-session -s mutt followed by echo $TERM results in xterm-256color
however,
tmux new-session -s mutt 'echo $TERM && sleep 10'
yields screen. To get around this, try:
tmux new-session -s mutt 'TERM=xterm-256color; echo $TERM && sleep 10'
to see the correct setting, and to make mutt work:
tmux new-session -s mutt 'TERM=xterm-256color mutt'
Best Answer
Tmux by default will "take over" text selection with a mouse, and trigger its own internal selection for use with its own cut and paste.
If you want to use the OS selection, so that you can use the OS cut and paste, hold down shift while selecting.
Then cut and paste will work normally.