What is the reason for requiring an SMTP server to relay e-mails?

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The reason for requiring an SMTP server to relay e-mails is that SMTP (Simple Mail Transfer Protocol) is the standard protocol used for sending and receiving email messages over the internet. An SMTP server is responsible for forwarding email messages from one server to another until they reach their destination. This is necessary because email messages often need to travel through multiple servers and networks before they reach the intended recipient. Without an SMTP server to relay emails, the messages would not be able to reach their intended recipients. Additionally, SMTP servers are also responsible for ensuring that email messages are properly formatted and authenticated to prevent spam and other unwanted messages from being sent.