Like every unix program that occasionally has cause to send email notifications,
mail assumes that there is a functioning MTA on localhost that is 1) capable of accepting mail and 2) knows how to pass it on.
To find out what mail server you're running, try
telnet localhost 25 and look at the identifier string.
mailq, if it exists for you, will show you what messages are currently in the local mail server's queue, possibly with an explanation as to why it hasn't been passed on to its destination yet.
In addition, most distributions by default configure MTAs and syslog to report mail log messages to either
/var/log/mail.log or similar. Look in
/var/log/ for any file that looks viable, and grep it for 'bar.com'
Without more information as to what's going on it's hard to offer better advice than this, sorry.