For security reasons, you've created a script on a server that monitors if non-domain users have been added to the local administrators group. You've added logic to the script that sends an email out every time the script is run.
Your script utilizes SMTP for its reporting. Select the port that you will need to ensure is open for your script to send emails.
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Answer Description
Port 25 is the port that SMTP utilizes to send emails. You will also need to list a functioning SMTP server in your script for this to work.
Wikipedia
The Simple Mail Transfer Protocol (SMTP) is an Internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-level email clients typically use SMTP only for sending messages to a mail server for relaying, and typically submit outgoing email to the mail server on port 587 or 465 per RFC 8314. For retrieving messages, IMAP (which replaced the older POP3) is standard, but proprietary servers also often implement proprietary protocols, e.g., Exchange ActiveSync.
SMTP's origins began in 1980, building on concepts implemented on the ARPANET since 1971. It has been updated, modified and extended multiple times. The protocol version in common use today has extensible structure with various extensions for authentication, encryption, binary data transfer, and internationalized email addresses. SMTP servers commonly use the Transmission Control Protocol on port number 25 (for plaintext) and 587 (for encrypted communications).
Simple_Mail_Transfer_Protocol - Wikipedia, the free encyclopedia 