Running a PHP program in a CGI Program written in C

Well, scalability is probably Python-sided if you use FastCGI. Otherwise, I would say PHP. One thing to keep in mind: if you're integrating with a C system, Python is FAR easier to write interfaces for. Python really allows you to build true systems and processes. php is an imbedded language that is fairly limited, while Python is a general purpose programming language. For something as complex as a web-based mail system I would say Python because it allows you to do so much more than php. But, if you aren't going to be doing more than what php would do anyhow, they're no reason to move to Python unless you just like it more.