smbclient authfile format

[Wood, Ally (ELS)]

I'm having trouble using an authfile to define my username, password and
domain.

This command is working fine:
smbclient <server> -U <username%password> -W <domain> -c 'put foobar'

However when I change it to use an authfile it fails with "session setup
failed: ERRDOS - ERRnoaccess (Access denied.)"
smbclient <server> -A my.conf -c 'put foobar'

Where my.conf contains the following text
username=myuser
password=mypass
domain=mydomain

I've tried spaces before and after the '=' sign and upper and lowercase
passwords but with no joy.

Should the authfile be in a special format rather than a flat file?

Thanks
Ally