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SMTP (Simple Mail Transfer Protocol)
SMTP (Simple Mail Transfer Protocol)
📧 Overview
- SMTP is a protocol used for sending emails over IP networks.
- It works in both:
- Client ↔ Mail Server (email client sending to a server).
- Server ↔ Server (mail transfer between SMTP servers).
🔌 Ports and Encryption
- Default Port: 25 (plain text).
- Secure Ports:
- 587: Authenticated users with STARTTLS (upgrades to encrypted).
- 465: SMTP over SSL/TLS.
- SMTP by default is unencrypted, transmitting data in plain text.
- STARTTLS is used to secure communication after connection starts.
✅ Authentication & Security
- Uses SMTP-Auth via ESMTP for authenticated sending.
- Only authorized users can send emails, preventing spam and abuse.
- Encrypted connections protect authentication data (username/password).
📤 Email Transmission Flow
- Mail User Agent (MUA): User’s email client.
- Mail Submission Agent (MSA): Checks validity (relay server).
- Mail Transfer Agent (MTA): Sends/receives email, checks for spam.
- Mail Delivery Agent (MDA): Delivers to recipient’s mailbox (IMAP/POP3).
- Flow: MUA ➞ MSA ➞ MTA ➞ MDA ➞ Mailbox
🚫 Challenges & Limitations
- No reliable delivery confirmation:
- SMTP returns basic English error messages if undelivered.
- Unauthenticated sender:
- Easy to spoof sender address.
- Open relays can be abused to send mass spam (Open Relay Attack).
🛡️ Security Techniques
- SPF (Sender Policy Framework): Verifies sender IP.
- DKIM (DomainKeys Identified Mail): Ensures message integrity.
- ESMTP: Modern extension that enables:
- STARTTLS (encryption after EHLO).
- AUTH PLAIN (safe authentication).
Default Configuration
cat /etc/postfix/main.cf | grep -v "#" | sed -r "/^\s*$/d"SMTP Commands
- AUTH PLAIN
- HELO
- EHLO
- MAIL FROM:
- RCPT TO:
- DATA
- RSET
- VRFY
- EXPN
- NOOP
- QUIT
Connect to SMTP Server via Telnet
telnet <IP> 25Send an Email via Telnet
EHLO inlanefreight.htb
MAIL FROM:<sender>
RCPT TO:<recipient>
DATA
<email headers and body>
.
QUITProxy Command
CONNECT <IP>:25 HTTP/1.0Open Relay Misconfiguration
mynetworks = 0.0.0.0/0SMTP (Simple Mail Transfer Protocol)
https://fuwari.vercel.app/posts/footprinting/smtp-simple-mail-transfer-protocol-1ded0918708080f69e80e08338428c26/