Techies: How do you send email through locahost?

what MTA are you running? postfix? sendmail?

http://www.authsmtp.com

you can run sendmail with local only, being it doesnt actually use port 25, but it lets you send mail. just no outside access can use it for smtp (spamming mostly).

what os, linux, freebsd, etc?

Definitely depends on the OS. Also there's a bit more to it than just a one liner response; particularly if you want ensure your mail is actually delivered.

1) You need to setup your mail.hostname.com DNS record to point to your box. I HIGHLY recommend using a separate IP address for this.

2) You should setup a reverse DNS record as well to avoid a lot of spam traps. AKA a PTR record. This is another reason you should use a completely separate IP address.

3) Configure your sendmail, qmail, postfix (your choice) server to recognize your domain that you will be sending mail from.

4) If you really want to ensure your mail gets delivered (and this goes for a lot of sponsors out there because I see a ton of mail going straight to spam) you should setup three very important email authentication services; Sender Policy Framework (SPF), Sender ID, and DomainKeys Identified Mail (DKIM). These service tell the big players' (and people with good mail servers) servers that you are legit and will make sure your mail actually lands in someone's inbox rather than their spam folder.

if you run your site through php, smtp server can be defined in php.ini and then when you do mail() in php it uses whatever your have defined in php.ini