Sending email to same/different domains

Question

I am doing a Networks course and as I was reading about SMTP I thought about the following matter:

If a user sends an email to user1@example.com, user2@example.com, user3@example.com, does it take the same amount of time to send the emails separately as it does to send it as a multi-recipient email?

Or when the first one is sent, the next two are easier to send since the connection is already established with that server? Or does the connection close immediately thus making no difference?

What about when the emails belong to different domains?

Answer 1

It depends on how the MTA is programmed. The SMTP protocol allows the sender to send multiple RCPT TO commands, to list all the recipients of a message, and any reasonably-designed MTA will make use of this to consolodate them.

For the multiple-domain case, your mail client will use a single connection to upload the message to its submission server. That server will then use one connection per destination domain for the recipients.