How two MTAs (Message Transfer Agent) communicate with each other using a single TCP connection--after establishing the connection to port 25, the sending mail server waits for the receiving mail server to talk first...

HELO... by giving its identity and telling whether or not it is prepared to receive mail--if not or Server-time-out the sending mail server terminate the connection and try again later and the undeliverable message will be returned with an error report rfc 821 containing the first part (message headers) to the sender.

Server-time-out - The sending and receiving mail server are busy and having different timeouts, when one of them gives up while the other is still trying--resulting in the connection being terminated unexpectedly. Or the "receiving or sending" mail server terminate the connection due to imposed limits by time or quantity of mails.

If the server is willing to accept email, the sending mail server announces whom the email is coming from Mail-FROM and whom it is going to RCPT-TO--if such a recipient exists at the receiving mail server--it gives the go-ahead DATA the email is delivered, a reply receipt acknowledged and QUIT.

A HELO command with the recipient domain, followed by.... Note: Command is a request for mail service action sent by the sender SMTP to the receiver SMTP.

...A Mail-FROM command and follows by the; from address of the email and...

A RCPT-TO command is issued, followed by the email address of the intended recipient and...

A DATA command is accepted by the recipient mail server and your ISP mail server proceeds to send the message and followed by...

A QUIT command to terminate the process.

If you have sent 100 emails to 100 different email domains, the above process is repeated with each and every one of your email recipients ISP mail server. For more info, see: How Email Works?