The default value for the TIdSMTP.Pipeline property has been changed from True to False. Numerous users have reported problems about sending messages when TIdSMTP tries to pipeline its commands to the server. Disabling pipelining always fixes the problem, so it is now disabled by default until it can be fixed.
Also, a new AllowPipeling property hass been added to TIdSMTPServer. This controls whether the response to the EHLO command includes the PIPELINING extension or not.