Outbound email data should be used to assist in spam scoring
Example: You and a customer have exchanged a series of emails back and forth and randomly an email will be held.
The email addresses and domains (non free email providers) you have a history of sending email to should be a factor in the spam scoring and significantly reducing the likelihood that mail from that domain (less so) and email address (more so) are quarantined.
Today, in Outlook and OWA, there is an option “Automatically add people I e-mail to the Safe Senders List”. See: http://support.office.com/article/Add-people-I-e-mail-to-the-Safe-Senders-List-D3F9FA3E-1EC7-42E2-A465-2FB5BC02F5BF
However, it may be a personal preference. Using this feature without periodically reviewing your safe list can be problematic with the frequency of phishing and spoofing.