AFAIK the /etc/aliases are used for local delivery e.g. mails to the "root" user should delivered to the "administrator" user.
Have you tried the sender_canonical_maps setting?
1. Modify your Postfix configuration file # vim /etc/postfix/main.cf
2. Add sender_canonical_maps = hash:/etc/postfix/sender_canonicalto your config
3. Create the given file # vim /etc/postfix/sender_canonical
4. Add icinga email@example.com (on Debian based Distributions nagios firstname.lastname@example.org) to that file
5. Modify the file permissions to ensure that only "root" can read/modify it # chmod 600 /etc/postfix/sender_canonical
6. Create the Postfix lookup table # postmap /etc/postfix/sender_canonical
7. Restart/Reload your Postfix daemon
When Icinga now sends a mail your should see that it is send by "email@example.com".
Some mail provider expect that the authenticated user e-mail address is the same as the sending e-mail address, so you can't send from user "firstname.lastname@example.org" a mail with "email@example.com" as sender address.