SMTP simply means Simple Mail Transfer Protocol.
It’s an application used by mail servers to send, and receive mail between email senders and receivers.
Without an SMTP server, your email wouldn’t reach its destination.
Once an email is sent it transforms into a string of code sent to the SMTP server. The SMTP server processes that code and passes on the message.
If the SMTP server weren’t there to process the message, the email wouldn’t reach the destination.