Description: When users try to register with your blog or change their passwords by entering their username and email, WordPress indicates that their password has been emailed to them, but it is never received.

Reason and Solution: WordPress uses the standard php mail function, which uses sendmail. No account information is needed. This is not generally a problem if you are using a hosting service, but if you are using your own box and do not have an SMTP server, the mail will never send. If you are using a *NIX box, you should have either postfix or sendmail on your machine; you will just need to set them up (Google for how-to's). If you do not want to go through setting up a complete mail server on your *NIX box you may find ssmtp useful -- it provides "A secure, effective and simple way of getting mail off a system to your mail hub". On a Windows machine, try a sendmail emulator like Glob SendMail.

More help can be found on the forums at: http://wordpress.org/support/topic.php?id=24981

As a plugin-based alternative, you could try Configure SMTP. It "Configure SMTP mailing in WordPress, including support for sending e-mail via SSL/TLS (such as GMail)."

Windows Host Specific: With the plugin mentioned above you can work around the issue of e-mails not being received.

Another alternative is to edit the class-pop3.php file in wp-includes, there is a variable, $MAILSERVER, which allows you to hard code your smtp server. However, wp-php.php in the root directory actually loops through depending on the author to determine a value to set the from address on the e-mail and defaults to the admin address particularly in the case of users registering.

This is fine if your admin address is a valid e-mail address. For example, if your admin e-mail is: admin@"yourhost.com", your smtp server must also be smtp."yourhost.com". If the domains do not match your mail will not send. (Your SMTP server may not require authentication however, your host in most cases still checks to make sure incoming SMTP traffic is still valid. This is to prevent spam.)

When using the plugin wpPHPMailer the same applies, and there is an option to hard code the from address.