Review email security setup with Exchange Online

We have DKIM and SPF running and have increased the sensitivity of SPAM and Phishing filters. However, recently my client had 2 customers receive phishing emails that "appeared" to have come from users on my client's domain. Although I am pretty versed in administering Exchang/EOL admin, I am not an Exchange SME; and could use some help to review our current config and determine if there is more we could be doing, or things we should do differently.