If you have a cPanel based server, adding an SPF record or domain keys entries for domains are so much easier than manually entering them.
cPanel provides the following scripts/commands to help us through this task:
# /usr/local/cpanel/bin/domain_keys_installer <cPanel_username>
# /usr/local/cpanel/bin/spf_installer <cPanel_username>
Note that this would add the records for all domains under this cPanel account. Meaning for all Addon domains too.