PHPMailer is a a full-featured e-mail transfer class for PHP, which is able to send email via sendmail, PHP mail(), QMail, or by SMTP.

Installation:
- Copy class.phpmailer.php into your php.ini include_path.
- If you are using the SMTP mailer then place class.smtp.php in your path as well,
- In the language directory you will find several files like phpmailer.lang-en.php. if you look right before the .php extension that there are two letters. These represent the language type of the translation file. For instance "en" is the English file and "br" is the Portuguese file. Choose the file that best fits with your language and place it in the PHP include path. If your language is English then you have nothing more to do.
- If it is a different language then you must point PHPMailer to the correct translation. To do this, call the PHPMailer SetLanguage method like so:
$mail->SetLanguage("br", "/optional/path/to/language/directory/");

Key Features of PHPMailer:
- Supports emails digitally signed with S/MIME encryption!
- Supports emails with multiple TOs, CCs, BCCs and REPLY-TOs
- Works on any platform.
- Supports Text & HTML emails.
- Embedded image support.
- Multipart/alternative emails for mail clients that do not read HTML email.
- Flexible debugging.
- Custom mail headers.
- Redundant SMTP servers.
- Support for 8bit, base64, binary, and quoted-printable encoding.
- Word wrap.
- Multiple fs, string, and binary attachments (those from database, string, etc).
- SMTP authentication.
- Tested on multiple SMTP servers: Sendmail, qmail, Postfix, Gmail, Imail, Exchange, etc.
- Good documentation, many examples included in download.