By default settings, Nagios will send email notifications as nagios@localhost, identified by $ADMINEMAIL$ macro.
If you want to change this behavior, you need to set the email address you want to use in the nagios.cfg file:
# ADMINISTRATOR EMAIL/PAGER ADDRESSES
# The email and pager address of a global administrator (likely you).
# Nagios never uses these values itself, but you can access them by
# using the $ADMINEMAIL$ and $ADMINPAGER$ macros in your notification
# commands.
admin_email=nagios@localhost
admin_pager=pagenagios@localhost