When attempting to use telnet with port 465 (SMTPS), you're encountering immediate disconnection because:

• Port 465 requires SSL/TLS encryption from the first byte
• Basic telnet doesn't support encrypted connections
• Modern SMTP servers enforce immediate security protocols

Instead of telnet, use these alternatives for port 465:

# Using openssl (recommended)
openssl s_client -connect smtp.gmail.com:465 -crlf -quiet

# Using stunnel (alternative)
stunnel -c -d localhost:2525 -r smtp.gmail.com:465
telnet localhost 2525

Here's how a successful session should look with openssl:

$ openssl s_client -connect smtp.gmail.com:465 -crlf -quiet
220 smtp.gmail.com ESMTP xxxxxxxxxxxxxx
EHLO example.com
250-smtp.gmail.com at your service
250-SIZE 35882577
250-8BITMIME
250-STARTTLS
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-CHUNKING
250 SMTPUTF8
AUTH LOGIN
334 VXNlcm5hbWU6
[base64-encoded username]
334 UGFzc3dvcmQ6
[base64-encoded password]
235 2.7.0 Accepted
MAIL FROM: <sender@example.com>
250 2.1.0 OK
RCPT TO: <recipient@gmail.com>
250 2.1.5 OK
DATA
354 Go ahead
Subject: Test email
From: sender@example.com
To: recipient@gmail.com

This is a test message.
.
250 2.0.0 OK
QUIT
221 2.0.0 closing connection

Authentication Errors: Ensure you've enabled "Less secure apps" in Google Account settings or use App Passwords if 2FA is enabled.

Connection Timeouts: Many ISPs block outbound port 465. Test with port 587 (submission) instead:

openssl s_client -starttls smtp -connect smtp.gmail.com:587 -crlf

For frequent testing, consider this bash script:

#!/bin/bash
{
  sleep 1
  echo "EHLO $(hostname)"
  sleep 1
  echo "AUTH LOGIN"
  sleep 1
  echo "$(echo -n 'your@email.com' | base64)"
  sleep 1
  echo "$(echo -n 'yourpassword' | base64)"
  sleep 1
  echo "MAIL FROM: <your@email.com>"
  sleep 1
  echo "RCPT TO: <recipient@domain.com>"
  sleep 1
  echo "DATA"
  sleep 1
  echo "Subject: Test Email"
  echo ""
  echo "This is an automated test email."
  echo "."
  sleep 1
  echo "QUIT"
} | openssl s_client -connect smtp.gmail.com:465 -crlf -quiet

When attempting to use telnet directly on port 465 (SMTPS), you'll encounter immediate disconnection because:

$ telnet smtp.gmail.com 465
Trying 108.177.126.108...
Connected to smtp.gmail.com.
Escape character is '^]'.
Connection closed by foreign host

Port 465 requires SSL/TLS encryption from the first byte - traditional telnet can't establish the secure layer. Here's what's happening under the hood:

1. The SMTP server expects TLS handshake immediately
2. Telnet sends plaintext commands instead
3. Server terminates the connection for security

Option 1: Use openssl instead of telnet

The proper way to manually test SMTP over port 465:

openssl s_client -connect smtp.gmail.com:465 -crlf -quiet

Example session:

EHLO example.com
AUTH LOGIN
[base64-encoded-username]
[base64-encoded-password]
MAIL FROM: <test@example.com>
RCPT TO: <recipient@gmail.com>
DATA
Subject: Test email
This is a test message.
.
QUIT

Option 2: Use port 587 with STARTTLS

For servers supporting STARTTLS (port 587):

openssl s_client -starttls smtp -connect smtp.gmail.com:587 -crlf

Option 3: Use swaks (Swiss Army Knife for SMTP)

Install and run:

swaks --to recipient@example.com \
--from test@example.com \
--server smtp.gmail.com:465 \
--tlsc \
--auth-user your@gmail.com \
--auth-password 'yourpass'

• Always check if your IP isn't blocked (common with Gmail)
• Enable "Less secure apps" if using Gmail (or use App Password)
• Verify firewall isn't blocking outbound 465
• Check server logs for connection attempts

Here's a complete test session with annotations:

# Establish connection
openssl s_client -connect smtp.example.com:465 -crlf -quiet

# Server responds with banner
220 smtp.example.com ESMTP Service Ready

# Identify yourself
EHLO client.example.com
250-smtp.example.com Hello [192.0.2.1]
250-SIZE 35882577
250-STARTTLS
250-AUTH LOGIN PLAIN
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-CHUNKING
250 SMTPUTF8

# Authenticate
AUTH PLAIN [base64-encoded-credentials]
235 2.7.0 Authentication successful

# Compose email
MAIL FROM:<sender@example.com>
250 2.1.0 Sender OK
RCPT TO:<recipient@example.com>
250 2.1.5 Recipient OK
DATA
354 Start mail input; end with <CRLF>.<CRLF>
Subject: SMTP/S Test
From: Sender <sender@example.com>
To: Recipient <recipient@example.com>

This message was sent via SMTPS.
.
250 2.0.0 OK 1234567890 qp12345

# Clean exit
QUIT
221 2.0.0 Closing connection