Transmission control protocol is an protocol which is used to transmit data from client to server.
When an email is sent then it is used some transmission protocols for sending and receiving the email which are SMTP, POP3(post office protocol), IMAP.
SMTP(simpel mail transfer protocol) is part of the application layer of the TCP/IP protocol. Using a process called "store and forward," SMTP moves your email on and across networks. It works closely with something called the Mail Transfer Agent (MTA) to send your communication to the right computer and email inbox.
This way they successfully transfer the emails with security many other methods are used like this
Here is an short intro of all this email protocols
So after clicking the submit button the email sent via smtp and receive on server or client side and vice versa
generally in case of html forms data or email is sent on local server or designated address of that server.