content transfer encoding

[digitalecartoon]

// Multipart Bericht            

$email_message .= "This is a multi-part message in MIME format.\n\n" . 
                    "--{$mime_boundary}\n" . 
                    "Content-Type:text/plain; charset=\"iso-8859-1\"\n" . 
                    "Content-Transfer-Encoding: 7bit\n\n" . 
                    $email_txt . "\n".
                    "--{$mime_boundary}\n" . 
                    "Content-Type:text/html; charset=\"iso-8859-1\"\n" . 
                    "Content-Transfer-Encoding: 7bit\n\n" . 
                    $email_htm ;

This is part of a html mail with alternative text version. I often see this Content-Transfer-Encoding: 7bit line in scripts like these and I'm wondering if it's necessary since 7bit is default? So why do a lot of people put it in?

[digitalecartoon]

Could I leave it out (since as far as I know 7 bit is default), or is there some reason why that line is often present in html mail scripts?
Here's an article by Kevin Yank about mixed html/text php mailing which also contains several 'content-transfer-encoding: 7-bit' lines:
http://www.sitepoint.com/article/advanced-email-php/4

[leatherback]

As far as I know, the 7bit encoding is the default for text. If you start adding attachments you need e.g., base64 encoding. I don't think you NEED to define it. But I think it won't hurt. That being said: I do not add it tomy headers, and have not had a lot of email gon-a-missin'...