Based on the command line options (
http://ant.apache.org/manual/index.html) your two best bets are:
1) Try the -k option
2) Write a CustomExitCode class and invoke the mail target from within it.
Of course, you could always create a script that does:
ant xxx yyy
ant mail
That way the mail target would always run. You'd probably have to tweak the mail target since it would be mailing the result from the prior ant run.
Another thought: having a "finally" target that runs before exit whether the build passed or failed is a good idea. If you created a patch to do that you could submit it back to the community.