On the Solaris system, the version of the OS? Use the command uname -a and post the output.
Run ps -ef (or ps -auxw) | grep mail - post the output
Look in /etc/syslog.conf to see if mail logs are maintained - if they are, look at the logs and see what errors are being posted. Post the errors.

If you are running Sendmail, you can use this command to see if there is a problem with the configuration.
/usr/lib/sendmail -d0.1 -bt < /dev/null