I have mail templates with placeholders for names (ex. Dear XXXXXX).
Is it possible to make apple mail check the content of my mail when sending and to stop an email from getting send out when it finds my placeholders?
Are you committed to using just Mail.app?
If you go to http://macupdate.com and search for Mail Merge there are a page of hits. At first blush, this one seemed to be right up your alley, but I may not understand fully what you are looking for.
https://www.macupdate.com/app/mac/11731/mailmergeapp Claims to use your address book, and a composed message, and it does the substituting.
If your outgoing messages are batched, take a look at external software for sending out email. In my other life I use Mail Chimp to send out news letters, but there are many many others, with various features.
A final possibility is to abandon Mail.app alltogether, and throw together a perl script that does the substitutions, and makes a call to /bin/mail for sending.