Postfix Main.cf, Config Functions

[RHITMacMan]

I've been trying to figure out a way to setup my postfix so that it will work properly with my ISP. I'm assigned a different IP and resolved hostname at each login. I get IPs like 1XX.XXX.XXX.136 and then a resolved name like abc136.users.domain.com. I have been putting "localhost" in the Postfix config file, but it's been causing problems with my ISP when mail is bounced. When a message is bounced, it doesn't know who localhost is and therefore who to bounce it back to. In particular, no mail coming from my computer using the mail() in PHP is accepted by my mac.com address, so it's bounced. Why? I'm not sure. I'm hoping that once I get a properly resolved name, it will like the mail and accept it. I'm guessing it thinks my mail is junk mail since it doesn't have a proper address, but I'm not sure. The following quote is located in the main.cf file..

# The myhostname parameter specifies the internet hostname of this
# mail system. The default is to use the fully-qualified domain name
# from gethostname(). $myhostname is used as a default value for many
# other configuration parameters.

My question is, can I use the gethostname() and other funtions within the main.cf file myself? If so, where can I get a list of these functions and what they do. I'd like to have Postfix use the dynamic resolved name while I'm online and then "localhost" when I'm not.