While "just" an example it still isn't nice to leak memory.

Bug: http://curl.haxx.se/bug/view.cgi?id=1368
Fixed-by: Marko

In the initializer for len, there is no prototype for "strlen".
In this statement, there is no prototype for "memcpy".

... since it is never returned since a long while back.

error: 'MULTI_PERFORM_HANG_TIMEOUT' undeclared

Simplified the SMTP multi example as this example should demonstrate
the differences the easy and multi interfaces rather than introduce new
concepts such as user authentication and TLS which are shown in the TLS
and SSL examples.

Updated to read data from a callback rather than from stdio as this is
more realistic to most use cases.

Replaced the use of CURLOPT_USERPWD for the preferred CURLOPT_USERNAME
and CURLOPT_PASSWORD options and used the same username and password for
all email examples which is the same as that used in the test suite.