It's possible that a strange conjunction of conditions may occasionally mean that cmail has trouble reactivating an existing XBoard. If this should happen, simply trying it again should work. If not, remove the file that stores the XBoard's PID (game.pid) or use the ‘-xreuse’ option to force cmail to start a new XBoard.
Versions of cmail after 2.16 no longer understand the old file format that XBoard used to use and so cannot be used to correspond with anyone using an older version.
Versions of cmail older than 2.11 do not handle multi-game messages, so multi-game correspondence is not possible with opponents using an older version.