When you ran telnet mail.mydomain.com 25 and received the following output:

Trying 127.0.0.1...
Connected to mail.mydomain.com.
Escape character is '^]'.

This indicates that:

• The Postfix service is running and listening on port 25
• The basic networking connectivity is working
• However, Postfix isn't responding with the expected SMTP banner

The issue likely stems from one of these scenarios:

# Check if Postfix is running
sudo systemctl status postfix

# Verify listening ports
sudo netstat -tulnp | grep postfix
# or alternatively
sudo ss -tulnp | grep postfix

Package updates sometimes reset configuration files. Verify these critical files:

# Main configuration file
sudo nano /etc/postfix/main.cf

# Check for these essential parameters:
myhostname = mail.mydomain.com
mydomain = mydomain.com
myorigin = $mydomain
inet_interfaces = all
mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain

After verifying the configuration, test SMTP with these commands:

# Basic SMTP test
telnet localhost 25
EHLO localhost
MAIL FROM: test@mydomain.com
RCPT TO: validuser@mydomain.com
DATA
Subject: Test email
This is a test message.
.
QUIT

# Check mail logs in real-time
sudo tail -f /var/log/mail.log

Ubuntu 11.04 might have these issues:

# Check firewall rules
sudo iptables -L

# Temporary disable firewall for testing
sudo ufw disable

# SELinux commands (if applicable)
sudo getenforce
sudo setenforce 0

For upgrade-related issues, try:

# Reconfigure postfix
sudo dpkg-reconfigure postfix

# Check package dependencies
sudo apt-cache depends postfix

# Reinstall if necessary
sudo apt-get install --reinstall postfix

Beyond telnet, consider these utilities:

# Using swaks for advanced testing
swaks --to user@example.com --server mail.mydomain.com

# Check DNS records
dig MX mydomain.com
dig A mail.mydomain.com

---

When you ran telnet mail.mydomain.com 25, the connection was established (as shown by "Connected to mail.mydomain.com"), but the SMTP banner didn't appear. This suggests Postfix is running but not responding properly. Here's what a healthy response should look like:

220 mail.mydomain.com ESMTP Postfix (Ubuntu)

Recent package updates might have affected:

• Configuration file locations or permissions
• Dependencies (like libssl or libdb)
• Service initialization

First check Postfix's status and logs:

sudo service postfix status
sudo tail -n 50 /var/log/mail.log

Common log errors post-upgrade include:

postfix/master[pid]: fatal: bind 127.0.0.1 port 25: Permission denied
postfix/postdrop: warning: unable to drop privileges: Permission denied

Check your main Postfix configuration:

postconf -n

Key parameters to verify:

inet_interfaces = all
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
smtpd_banner = $myhostname ESMTP $mail_name

Check if Postfix is actually listening on port 25:

sudo netstat -tulpn | grep :25
sudo ss -tulpn | grep :25

You should see output similar to:

tcp   0   0 0.0.0.0:25   0.0.0.0:*   LISTEN   1234/master

After verifying the above, try a full SMTP conversation:

telnet localhost 25
EHLO example.com
MAIL FROM: <test@example.com>
RCPT TO: <youruser@mydomain.com>
DATA
Subject: Test email
This is a test message.
.
QUIT

For Ubuntu 11.04 (now EOL), you might encounter:

• Obsolete dependency chains
• Incompatible library versions
• Missing configuration during upgrade

Consider checking package integrity:

sudo dpkg --verify postfix
sudo apt-get install --reinstall postfix

For comprehensive testing, use swaks (Swiss Army Knife for SMTP):

swaks --to user@mydomain.com --from tester@example.com --server mail.mydomain.com

This provides detailed SMTP transaction logging and is more reliable than manual telnet tests.