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SMTP Submission
Port 587 is primarily used for the submission of email messages over the Simple Mail Transfer Protocol (SMTP). It is designated for client-to-mail server communication, allowing authenticated users to send outgoing emails securely, typically requiring authentication and optional encryption. This port replaces legacy port 25 for authenticated message submission to improve security and adherence to email standards..
Overview:
Port 587 is the default port for email message submission using SMTP. It is designed to facilitate the sending of outgoing mail from client devices or applications to email servers prior to relaying or further delivery. Unlike port 25, which is mainly used for server-to-server transmission (SMTP relay), port 587 enforces authentication, making it more appropriate for sending emails from end-users.
Protocol Details:
Port 587 supports the SMTP protocol with extensions specified in RFC 6409, which defines message submission. It generally requires the use of the STARTTLS command to upgrade from an unencrypted to an encrypted connection, ensuring confidentiality and integrity during transmission. Clients must authenticate via mechanisms such as LOGIN, PLAIN, or newer methods to submit messages successfully.
Usage Scenarios:
This port is the recommended submission port for email clients (e.g., Outlook, Thunderbird, mobile email apps) when sending mail. Many ISPs and hosting providers enforce message submission on port 587 with STARTTLS to meet best practices and enhance security without obstructing legitimate email sending activities.