I am having trouble sending email to my ISP's smtp server from the web client. My ISP is expecting a plain username and password.

My postfix main.cf tail

smtpd_sasl_auth_enable = yes
smtpd_tls_auth_only = yes
disable_dns_lookups = yes
message_size_limit = 10240000
relayhost = smtp.mydomain.com
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/opt/zimbra/conf/relay_password
smtp_sasl_mechanism_filter = plain
smtp_tls_enforce_peername = no
smtp_use_tls = no

Username and password in 'relay_password' is correct

Error in var/log/zimbra.log

Dec 28 19:53:04 zimbra postfix/smtp[29914]: warning: SASL authentication failure: No worthy mechs found
Dec 28 19:53:04 zimbra postfix/smtp[29914]: DBE0A1040741: to=, relay=smtp.mydomain.com[XXX.XXX.XX.XXX], delay=6, status=deferred (Authentication failed: cannot SASL authenticate to server smtp.mydomain.com[XXX.XXX.XX.XXX]: no mechanism available)

Thanks in advance for your help
btw - Zimbra is impressive thus far