When attempting SMTP authentication between Postfix and Dovecot, the mail server fails with critical errors:

postfix/smtpd[5278]: warning: SASL: Connect to smtpd failed: No such file or directory
postfix/smtpd[5278]: fatal: no SASL authentication mechanisms

First, let's validate the key configuration files:

Postfix main.cf

smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
smtpd_sasl_auth_enable = yes
smtpd_sasl_authenticated_header = yes

Dovecot 10-master.conf

unix_listener /var/spool/postfix/private/auth {
    mode = 0666
    user = postfix
    group = postfix
}

1. Socket Path Mismatch

The most frequent issue is path inconsistency between Postfix and Dovecot configurations. Verify:

# For Postfix chroot environment
ls -l /var/spool/postfix/private/auth

# Should show:
srw-rw-rw-. 1 postfix postfix 0 Feb 23 22:46 /var/spool/postfix/private/auth

2. Permission Issues

Even with correct paths, permissions can block communication:

chmod 0660 /var/spool/postfix/private/auth
chown postfix:postfix /var/spool/postfix/private/auth

3. SASL Mechanism Verification

Check available mechanisms with:

postconf -a
# Should output:
cyrus
dovecot

Forcing Verbose Logging

Add these to /etc/dovecot/conf.d/10-logging.conf:

auth_verbose = yes
auth_debug = yes
auth_debug_passwords = yes

Testing Authentication Directly

Verify Dovecot auth works independently:

doveadm auth username
# Enter password when prompted

Postfix Test Commands

Essential diagnostic commands:

postconf -M | grep smtpd
postconf -n | grep sasl
telnet localhost 25

Here's a working minimal configuration:

Postfix (main.cf)

smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous
smtpd_sasl_local_domain = $myhostname
smtpd_recipient_restrictions = 
    permit_mynetworks,
    permit_sasl_authenticated,
    reject_unauth_destination

Dovecot (10-master.conf)

service auth {
    unix_listener /var/spool/postfix/private/auth {
        mode = 0660
        user = postfix
        group = postfix
    }
}

• Verify SELinux contexts if enabled: restorecon -Rv /var/spool/postfix/private
• Check process ownership: ps aux | grep postfix and ps aux | grep dovecot
• Confirm service status: service dovecot restart && service postfix restart

---

When trying to authenticate SMTP connections through Dovecot SASL with Postfix, you encounter the following critical error:

Feb 23 22:35:36 localhost postfix/smtpd[5278]: fatal: no SASL authentication mechanisms
Feb 23 22:35:36 localhost postfix/smtpd[5278]: warning: SASL: Connect to smtpd failed: No such file or directory

First, let's verify the key configuration elements in both Postfix and Dovecot:

Postfix main.cf Settings

smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
smtpd_sasl_auth_enable = yes
smtpd_sasl_authenticated_header = yes

Dovecot 10-master.conf

unix_listener /var/spool/postfix/private/auth {
    mode = 0666
    user = postfix
    group = postfix
}

Before proceeding with solutions, perform these essential checks:

1. Socket File Existence and Permissions

ls -l /var/spool/postfix/private/auth
# Should show:
# srw-rw-rw-. 1 postfix postfix 0 Feb 23 22:46 /var/spool/postfix/private/auth

2. SASL Mechanism Availability

postconf -a
# Should list:
# cyrus
# dovecot

Based on the error and configurations, here's the comprehensive solution approach:

1. Correct the smtpd_sasl_path Parameter

In your postconf output, there's a typo in smptd_sasl_path (missing 'd'). Fix it:

postconf -e "smtpd_sasl_path=private/auth"

2. Verify Dovecot SASL Socket

Ensure the socket path matches between Dovecot and Postfix:

# In dovecot configuration:
unix_listener /var/spool/postfix/private/auth {
    mode = 0660  # More secure than 0666
    user = postfix
    group = postfix
}

# In postfix main.cf:
smtpd_sasl_path = private/auth

3. Restart Services with Correct Order

The proper service restart sequence matters:

service dovecot restart
service postfix restart

If issues persist, try these advanced debugging methods:

1. Enable Verbose Logging

# In /etc/dovecot/conf.d/10-logging.conf:
auth_verbose = yes
auth_debug = yes

2. Test SASL Mechanism Directly

testsaslauthd -u username -p password -s smtp
# Should return: 0: OK "Success."

3. Verify Postfix-Dovecot Communication

telnet localhost 25
EHLO localhost
# Should show SASL mechanisms in response

Ensure all these elements are properly configured:

• Correct socket path in both services
• Proper permissions on auth socket (0660)
• Matching ownership (postfix:postfix)
• No typos in configuration parameters
• SASL mechanisms properly listed in postconf -a