email - Why shouldn't I use PHP's mail() function?

The general opinion when it comes to sending email messages in PHP is to stay clear of PHP's built-in mail() function and to use a library instead.

What I want to know are the actual reasons and flaws in using mail() over a library or extension. For example, the commonly specified headers that aren't included in a standard mail() call.

Answer

Quoting:

Disadvantages of the PHP mail() function

In some cases, mails send via
PHP mail() did not receive the
recipients although it was send by WB
without any error message. The most
common reasons for that issue are
listed below.

• wrong format of mail header or content
  
  (e.g. differences in line break
  between Windows/Unix)


• sendmail not
  installed or configured on your server
  (php.ini)


• the mail provider of the
  recipeint does not allow mails send by
  PHP mail(); common spam protection

Errors in the format of header or
content can cause that mails are
treated as SPAM. In the best case,
such mails are transfered to the spam
folder of your recipient inbox or send
back to the sender. In the worst case,
such mails are deleted without any
comment. If sendmail is not installed
or not configured, no mails can be
send at all.

It is common practice by free mail
provider such as GMX, to reject mails
send via the PHP function mail(). Very
often such mails are deleted without
any information of the recipient.