My setup is a single server that runs a few various PHP applications that will at times send email to users. All of our personal email is handled outside of the server but I wanted to know what options are there for a lightweight send only mail service with DKIM?

I'm trying to trim down the total number of services I'm running at any one time so if there is something that is an all in one solution that would be preferable.

postfix + opendkim

Thanks for the recommendation, I'll try it out.