Pipelining is an SMTP protocol extension aiming to improve SMTP performance when sending e-mail to multiple recipients by making it possible to send multiple commands without having to wait for a response after sending each of them. See a quick summary or the full specification if you are interested in details.
Chunking is another SMTP protocol extension that defines an alternative to DATA command suitable for sending large messages. It makes it possible to send MIME messages that use binary transfer encoding, it eliminates the need to scan messages for "CR LF . CR LF" sequences upon sending and receiving to detect the end of message, and it makes it easier to gracefully abort sending large messages while they are being uploaded. See the full specification for details.
Unfortunately, some proxy servers, firewalls and antivirus software that inhibits and modifies SMTP communication interfere with these two extensions - they let the server announce to the clients that these extensions are supported, even though they are not actually handled properly by the the proxy/firewall/antivirus.
The line of code disables both of these extensions, making the
Smtp object use the
old-style SMTP protocol that is a bit more compatible.
Interestingly, we have actually been considering disabling these extensions by default some years ago. However, in the meantime, problems with SMTP-aware software diminished (most vendors fixed their issues) and we finally decided to keep them. Unfortunately, your experience proves that their functionality is still not guaranteed.