We recommend multiple e-mail templates per campaign so that it is not easy to identify as the same message. If you include {subject} in the e-mail it will automatically put in the subject of the post in the e-mail.
E.x.:
Hi, I saw your ad: {subject}.
This will help change up some of the e-mails.
Also it is recommended not to use the same URL in each e-mail message, we actually recommend using multiple url shorteners, like tinyurl and so forth so that each template can have a unique URL that forwards to the same page.
Note, usually Craigslist does not block this type of pattern but in many cases free email account services such as gmail will identify it and block it after many repeated messages. Change templates slightly every few days and change URLs with url shortners often (note url shortners can give new URLs if you capitalize some of the letters in your url, e.x.):
http://www.craigslistemailer.com -> tinyurl.com/abcd
http://WWW.crAigslistemailer.com -> tinyurl.com/defg
Even though they are teh same url, they will have unique tinyurls because of the uppercase/lowercase difference.