I'm writing an email notification system that needs to send many hundreds of emails at once. The system needs to be deployed on various setups with different MTAs. Depending on the configuration, some MTAs seem to receive an email from an application and queue it as fast as possible; others will hold the sender application while processing (sending?) the message – this would cause the application to block for extended periods of time.
Would it be reasonable to expect MTAs on production machines with modern software to queue mail quickly (or be configured to do so)?