503 5.5.1 Error: authentication not enabled

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503 5.5.1 Error: authentication not enabled is an error message returned by Postfix.

Another symptom is the absence of any "AUTH" lines in the response from Postfix when the client sends EHLO:

not working	working
EHLO hypertwins.org 250-(redacted).com 250-PIPELINING 250-SIZE 10240000 250-VRFY 250-ETRN 250-STARTTLS 250-ENHANCEDSTATUSCODES 250-8BITMIME 250 DSN AUTH LOGIN 503 5.5.1 Error: authentication not enabled	EHLO hypertwins.org 250-(redacted).com 250-PIPELINING 250-SIZE 10240000 250-VRFY 250-ETRN 250-STARTTLS 250-AUTH PLAIN LOGIN 250-AUTH=PLAIN LOGIN 250-ENHANCEDSTATUSCODES 250-8BITMIME 250 DSN AUTH LOGIN 334 VXNlcm5hbWU6

This message can appear even when smtpd_sasl_auth_enable is set to "yes" in main.cf.

Notes

The changes suggested here seemed to work, albeit with some modifications for my particular config:

## 2011-07-31 trying this, per http://forums.gplhost.com/phpBB2/image-vp3985.html , in attempt to fix smtp external auth
smtp_sasl_auth_enable = no
smtpd_sasl_security_options = noanonymous
smtpd_sasl_local_domain = /etc/mailname
smtpd_sasl_auth_enable = yes
smtpd_tls_auth_only = no

### OVERRIDE -- required in order to authenticate with Frontier's SMTP server/relay
smtp_sasl_auth_enable = yes

## (wzl added 2010-07-04)
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/dovecot-auth
smtpd_sasl_authenticated_header = yes
smtpd_sasl_local_domain = $myhostname
broken_sasl_auth_clients = yes
smtpd_recipient_restrictions = reject_unknown_sender_domain, reject_unknown_recipient_domain, reject_unauth_pipelining, permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination 
smtpd_sender_restrictions = reject_unknown_sender_domain

## more SASL/TLS stuff (was at the end of main.cf, moved here 2011-07-31)
smtp_sasl_authentication_filter =
myorigin = (redacted).com
smtpd_tls_security_level = may
smtpd_sasl_security_options = noanonymous