SPF record question- multiple IP addresses

I have been having problems with deliverability of email messages sent from my server. I have several IPs on it, one being the main server IP xxx.15 and another for a specific domain on xxx.16. The dot 16 IP has most of the shared accounts on it but the mail server is on dot 15.

I discovered that when I create new accounts it is creating SPF records that only list the xxx.16 IP address as being allowed to send mail for the domain. I have now added the xxx.15 IP to the SPF record and am hoping this will help things (let me know if I have this all screwy in my head).
My question is, how can I set up cpanel to add that xxx.15 address automatically when it sets up a new account?

 

Thank you NixTree for your help and suggestions. I'm my own administrator, learning fast, but still a little clueless at times. Could you clarify the following for me:

1) I am looking in my Edit Zone Templates (simple) section in WHM and was going to put that line in but then had a thought, since the system already adds the xxx.15 address if I put it in like that (with both ips) will it cause any problems due to the duplication?

2) Do I also need to add the same line to the standard and standardvirtualftp zone templates as well?

3) Also, once I add that record to the template what do I need to do to propagate it to all the accounts on the server or should I go through and change things one at a time for the existing accounts?