|Date: December 5, 2009
| Pingpong is just my (Daniel's) jestful collective name on the protocols that
| share a very similar kind of back-and-forth procedure with command and
| responses to and from the server. FTP was previously the only protocol in
| that family that libcurl supported, but when POP3, IMAP and SMTP joined the
| team I moved some of the internals into a separate pingpong module to be
| easier to get used by all these protocols to reduce code duplication and ease
| code re-use between these protocols.
| In 7.20.0 we converted code to use the new pingpong code from previously
| having been all "native" FTP code.
| There's no support in the documented URL format to specify the exact mail to
| get, but we support that as the path specified in the URL.
| There's no official URL syntax defined for SMTP, but we use only the generic
| one and we provide two additional libcurl options to specify receivers and
| sender of the actual mail.