pls. provide your current log - file ( "/var/log/mail.log" / "/var/log/maillog" ) - entries and the correponding configuration files ( main.cf + master.cf from "/etc/postfix" ), if you experience further issues/errors/problems, after following the suggestions from the already provided forum - link. People willing to help you should have something to investigate based on facts and describing the issues/errors/problems might not be sufficient "to guess" your root cause.
In Outlook, many people don't check the "Outgoing mail server requires authentication" checkbox, because it is not obviously displayed in the assistant dialogue. As in the default setting, Outlook does not authenticate at the SMTP server, relay access will be denied. Simply checking the box will probably solve the problem.