When configuring Postfix as a relay through your hosting provider's SMTP server, the SASL (Simple Authentication and Security Layer) authentication failure with "no mechanism available" typically indicates a mismatch between the authentication methods supported by your Postfix configuration and what the remote SMTP server expects.

From your configuration, I notice several potential problems:

# Problematic setting in your postconf output:
smtp_sasl_auth_enable = no  # This should be 'yes' for relay authentication
smtp_sasl_type = cyrus      # This might need to be changed

First, let's correct the fundamental SASL configuration:

# Update your main.cf with these critical settings:
smtp_sasl_auth_enable = yes
smtp_sasl_type = dovecot    # Or 'cyrus' if you prefer
smtp_sasl_mechanism_filter = plain, login
smtp_sasl_password_maps = hash:/etc/postfix/sasl/sasl_passwd
smtp_sasl_security_options = noplaintext, noanonymous

On CentOS 7, ensure you have the proper SASL implementation:

sudo yum install cyrus-sasl cyrus-sasl-plain cyrus-sasl-lib
sudo systemctl restart postfix

To test available mechanisms, use this telnet command:

telnet mail.myhosting.com 587
EHLO example.com
QUIT

Look for "AUTH" in the server's response to see supported mechanisms.

Here's a verified working configuration for similar setups:

# /etc/postfix/main.cf additions:
relayhost = [mail.myhosting.com]:587
smtp_use_tls = yes
smtp_tls_security_level = encrypt
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_sasl_mechanism_filter = plain, login
smtp_tls_CAfile = /etc/ssl/certs/ca-bundle.crt

# /etc/postfix/sasl/sasl_passwd:
[mail.myhosting.com]:587    username:password

# Set proper permissions:
sudo chmod 600 /etc/postfix/sasl/sasl_passwd
sudo postmap /etc/postfix/sasl/sasl_passwd

Use this command to test email sending:

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

Check logs with tail -f /var/log/maillog for detailed error messages.

If issues persist, try these diagnostics:

# Check SASL capabilities:
postconf -n | grep sasl

# Test SASL authentication directly:
testsaslauthd -u username -p password -s smtp

# Verify TLS connection:
openssl s_client -connect mail.myhosting.com:587 -starttls smtp

---

The error message "SASL authentication failed; cannot authenticate to server [host]: no mechanism available" typically occurs when Postfix fails to negotiate a compatible authentication method with your SMTP relay server. This often happens due to misconfigured SASL parameters or missing authentication modules.

# Verify your SASL configuration
postconf -n | grep sasl
# Check available SASL mechanisms
saslauthd -v

First, ensure you have the necessary SASL libraries installed:

yum install cyrus-sasl cyrus-sasl-plain cyrus-sasl-md5

Modify your main.cf with these essential parameters:

smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_sasl_mechanism_filter = plain, login
smtp_tls_security_level = encrypt
smtp_sasl_tls_security_options = noanonymous

Ensure your sasl_passwd file follows this exact format (including square brackets for the server):

[mail.myhosting.com]:587    username:password

Then create the hash database:

postmap /etc/postfix/sasl/sasl_passwd
chmod 600 /etc/postfix/sasl/sasl_passwd /etc/postfix/sasl/sasl_passwd.db

Use this command to test authentication before sending actual emails:

echo "Test email" | mailx -s "Test Subject" -S smtp-use-starttls \
-S smtp-auth=login -S smtp=smtp://mail.myhosting.com:587 \
-S from="contact@mysite.com" -S smtp-auth-user=contact@mysite.com \
-S smtp-auth-password=yourpassword recipient@gmail.com

Enable verbose logging to troubleshoot authentication issues:

# In /etc/postfix/main.cf
debug_peer_level = 2
debug_peer_list = mail.myhosting.com

Then check logs with:

tail -f /var/log/maillog | grep sasl

• Missing square brackets around the server name in sasl_passwd
• Incorrect file permissions on sasl_passwd files
• Not running postmap after editing sasl_passwd
• Firewall blocking outbound connections to port 587
• Incorrect TLS configuration (try adding smtp_tls_note_starttls_offer = yes)

If plain auth fails, try configuring OAuth2 or other mechanisms:

# In /etc/postfix/sasl/smtpd.conf
pwcheck_method: auxprop
auxprop_plugin: sql
mech_list: PLAIN LOGIN CRAM-MD5 DIGEST-MD5
sql_engine: mysql
sql_hostnames: 127.0.0.1
sql_user: postfix_user
sql_passwd: password
sql_database: postfix_db
sql_select: SELECT password FROM mailbox WHERE username = '%u'