SMTP stands for Simple Message Transfer Protocol and it’s what your email client uses to send your messages to your server, as well as sending the messages to the mailbox of your recipient’s email server. Please note that SMTP is a delivery protocol only. The email is "pushed" to a destination mail server and it’s routed based on the destination server, not the individual user to which it is addressed. From the destination server, another protocol will push the email to the intended recipient.
Server administrators need to impose some control on which clients can use the serves to prevent unwanted behaviors such as spam. Most SMTP servers typically require authentication of clients by credentials before allowing access, which is why you need to enter your credentials to our system.
For more information about setting up your email notifications, please check out this article.