Thank - you for the reply. I did manage to solve the problem lastnight.
For me, this error is generated because I never used the shortname when creating my email account.
Meaning : When you create a login account (at least on the mac mini) you have a shortname and a long name.
For example, if you create a login account to your mac mini with name John.Doe, and short name 'John' .
Postfix picked up only the shortname. I noticed in the /varl/log/mail.log references to firstname.lastname@example.org.
Dropping my email account and recreating with shortname enabled me to receive in my yahoo account.
Without this modification I received delivery failure errors.
Ideally I would like to have an email account called John.email@example.com, but I have to investigate if some kind of alias solution is available.
I'm new to the Mac work, so any advise is appreciated.
macmini dovecot: auth: Error: od[getpwnam_ext]: No record for user