In the event that your CS account is closed and your email still needs to be forwarded somewhere, the CS staff can set up a mail alias for you that will automatically forward mail addressed to you to another address. Just email staff@cs.umd.edu if you need this done.
Mail aliases are not maintained indefinitely after you leave. You should plan to wean people off the old .cs.umd.edu email address as soon as your CS accounts go away. After you have been gone a long time (usually measured in years), we'll offer to remove an old alias, but can't keep updating it forever.
Note that users who graduate with a graduate degree are entitled to an alumni account, which may in fact preserve email forwarding indefinitely.