Simple Mail Transfer Protocol (SMTP) is a way in which email travels across the internet. An SMTP relay is a mail server that passes on your email message to another server that can transfer your message to the intended recipient. Email providers like Gmail own and manage SMTP servers; some allow you to connect to their servers directly while others require you to send email via their webmail applications. In the latter case, providers are also safeguarding against the risk of companies sending several emails in a short period of time and engaging in spamming.
Providers that allow direct access to their SMTP servers may or may not support SMTP relaying. ‘Support’ means that you can connect to their SMTP server to send outbound email to recipients whose email is not managed by the provider (e.g., they handle email for luxsci.net addresses but not yahoo.com).
SMTP authentication versus Secure SMTP
To avoid the risk of hackers spamming users, many email providers require authentication (e.g., via a username and password) to use their SMTP servers. Some providers may go beyond SMTP authentication and offer Secure SMTP, encrypting the communication between your computer and their server using SSL/TLS protocols. This way, the contents of your email message cannot be read along the transmission channel to the SMTP relay server.
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