What is the main characteristic of SMTP's push model?

In the push model of SMTP (Simple Mail Transfer Protocol), the primary characteristic is that email messages are transmitted directly from the sender's server to the recipient's server. This process occurs as soon as the sender sends the email, initiating a connection to the recipient's email server to deliver the message. This direct transfer emphasizes the proactive nature of the push model, where the sender's server actively sends the information rather than waiting for the recipient to request it.

The other choices represent different methodologies or functionalities. For example, retrieving messages from the recipient's server pertains to a pull model, where users check for messages rather than having them sent automatically. Storing messages until requested also aligns with a pull model scenario. Lastly, sending messages in batches might refer to certain operational practices to manage load but does not reflect the defining characteristic of how SMTP processes message delivery.