When configuring Sendmail to use an external SMTP server (commonly called a "smart host"), the error Host unknown (Name server: [IP]: host not found) typically indicates DNS resolution issues. Even though you can ping the IP address, Sendmail performs additional verification steps.

For CentOS 5 (also applicable to modern systems with adjustments):

# Edit sendmail.mc
define(SMART_HOST', [10.1.1.30]')dnl  # Brackets bypass DNS lookup
define(RELAY_MAILER_ARGS', TCP $h 587')dnl  # For Exchange port
define(ESMTP_MAILER_ARGS', TCP $h 587')dnl

For authenticated relays (Exchange often requires this):

# Create auth info file
echo 'AuthInfo:10.1.1.30 "U:your_username" "P:your_password" "M:PLAIN"' > /etc/mail/authinfo
makemap hash /etc/mail/authinfo.db < /etc/mail/authinfo
chmod 600 /etc/mail/authinfo*

After rebuilding Sendmail configuration:

make -C /etc/mail
service sendmail restart
echo "Test" | mail -s "SMTP Test" recipient@domain.com

• Check connectivity: telnet 10.1.1.30 25
• Verify DNS: host 10.1.1.30 and reverse lookup
• Inspect logs: tail -f /var/log/maillog

For complex Exchange environments:

define(SMART_HOST', mail.company.com')dnl
define(confAUTH_MECHANISMS', EXTERNAL GSSAPI DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')dnl
FEATURE(authinfo', Hash -o /etc/mail/authinfo.db')dnl

When configuring Sendmail to use an external SMTP server as a relay host, the error Host unknown (Name server: 10.1.1.30: host not found) typically indicates a DNS resolution problem. Even though the IP is pingable, Sendmail performs additional hostname verification.

Here's how to properly configure Sendmail to relay through your Exchange server:

# Edit your sendmail.mc file
FEATURE(nocanonify', canonify_hosts')dnl
define(SMART_HOST', [10.1.1.30]')dnl
define(confDOMAIN_NAME', yourdomain.com')dnl
define(confDELIVERY_MODE', background')dnl

The square brackets around the IP address tell Sendmail to skip DNS lookups:

• [10.1.1.30] - Direct IP connection without hostname resolution
• FEATURE(nocanonify') - Prevents Sendmail from modifying addresses
• Ensure your mail server accepts relaying from this IP

If your Exchange server requires authentication:

# Create/Edit /etc/mail/authinfo
AuthInfo:10.1.1.30 "U:yourusername" "P:yourpassword" "M:LOGIN"

# Generate the auth database
makemap hash /etc/mail/authinfo.db < /etc/mail/authinfo

1. Rebuild your Sendmail configuration:

   make -C /etc/mail

2. Restart Sendmail:

   service sendmail restart

3. Test your configuration:

   echo "Test" | mail -s "Test Subject" recipient@example.com

If emails still don't go through:

• Check /var/log/maillog for detailed error messages
• Verify network connectivity: telnet 10.1.1.30 25
• Test SMTP authentication separately if configured

Remember to adjust firewall settings if necessary to allow outbound SMTP traffic (port 25) to your Exchange server.