Generally speaking, your Macintosh email clients, like Mail or Thunderbird, will setup postfix for you. But you can do-it-yourself using /Applications/Utilities/Terminal.app
The following shows how to setup "postfix" on a Mac OS X system (like Tiger). To make things simple, I recommend starting a "bash" shell using the "sudo" command to make yourself "root" for the commands which follow. Be sure to "exit" at the end. Note that I'm using generic names for things like your host or your administrator's username. You would use your actual names and passwords. First, you must get into Terminal.app, stop any currently executing postfix, and replace its control file:
At this point you've stopped any prior version of postfix, and have replaced the org.postfix.master.plist file which controls postfix. You'll restart postfix at the end. Meanwhile, you need to examine the /etc/postfix/main.cf file, the postfix configuration information. The command which follows shows what I had in my version. Yours will most likely be different. You may have to edit it to redefine settings like relayhost, myorigin, and myhostname. In this example, "relayhost" specifies a Gmail smtp server and associated port. The assumption is that you are using a secure smtp server.
If you are NOT using a secure smtp server, then you can comment-out the last four (4) smtp_ entries above, something like this:
There's a "postswap" attachment that you can download that does most of this for you. Read the procedure below for a better understanding. But you must edit "postswap" so all "applyit smtp..." calls have the proper mail server address and :port, and your email address at the server is correct. You also should edit /etc/postfix/main.cf, ending it with:
It doesn't matter much what you put in myorigin, myhostname, and relayhost because you'll change them with "postswap". It is IMPORTANT that any non-#comment versions of ANY definitions between ###Start and ###End be deleted from the earlier portion of main.cf, so be careful. You'll have to "sudo" the editing because main.cf is owned by root. See the recommendation in the paragraph which follows.
I recommend "vi" or "vim" for the editing. Use /variable to find the variable that needs to be checked. Then edit that line. Uncomment the "smtp_sasl_..." variables by eliminating leading # characters. When you are done, save your results with ":wq". Then do the following command:
If you are using a secure smtp server, you need to set /etc/postfix/sasl_passwd to your "smtp.server_name:port email_address:password", such as the following:
Use the actual smtp.server_name (and :port if needed) that corresponds to your email account.
MyName is your account name on that smtp.server; and MyPassWord9 should be the corresponding password.
Lastly, you must reload the postfix control file and start postfix again:
You are done. Postfix should now be running and be able to send mail to your smtp server. You can now use "postswap" to change it very easily.
SPECIAL NOTICE: I've recently switched to using a new mail service, where I "send" the mail to be delivered. This company didn't allow a "localhost" to send mail. Most Macintosh systems are defined with a localhost name of the form: "MacName-MyName.local". Look in System Preferences under Sharing to see your Computer Name definition. It would look like "MacName-MyName" but with your computer's name and our name, and without the ".local" suffix. The "myorigin" term in main.cf can be any host name OTHER THAN a .local name. So what I did in "postswap" is test for this condition and set "myorigin=noreply.net". That's a valid Domain name, and is accepted by my new email server (and any other mail servers), and "noreply.net" tells the recipient they can't reply to email sent via "postfix" from your computer with a "mail" command.