The simplest route would be to change your local login to bobt, but since you don't want to do that we can get sendmail to use its rewrite rules to do this for you. You're going to need m4 to do this so make sure that the sendmail-cf RPM is installed (rpm -q sendmail-cf). Now here's the procedure:
In /etc/sendmail.mc, add the following lines:
define(`SMART_HOST',`mail.isp.net')dnl MASQUERADE_AS(isp.net)dnl FEATURE(`allmasquerade')dnl FEATURE(`masquerade_envelope')dnl FEATURE(genericstable, `hash -o /etc/mail/genericstable')dnl GENERICS_DOMAIN_FILE(`/etc/mail/genericsdomain')dnl MAILER(local)
Then run m4 on it:
m4 /etc/sendmail.mc >/etc/_sendmail.cf mv -f /etc/_sendmail.cf /etc/sendmail.cf
Now create your /etc/mail/genericstable and add the following line to translate your local address to your desired address:
Now convert /etc/mail/genericstable to a hashed database:
makemap hash genericstable.db < genericstable
Now create your /etc/mail/genericsdomain and add your fully qualified host name (type "hostname -f" to find out) to the genericsdomain file.
Restart sendmail with: /etc/rc.d/init.d/sendmail restart.