Outlook email IMAP issues

I've followed the instructions to bring my domain emails to my outlook application.  I can send and receive to other accounts on my domain, and can receive from any other email, but keep getting return to sender messages when sending outside of our domain.  

"

SMTP error from remote mail server after end of data:

    550-5.7.26 This mail is unauthenticated, which poses a security risk to the

    550-5.7.26 sender and Gmail users, and has been blocked. The sender must

    550-5.7.26 authenticate with at least one of SPF or DKIM. For this message,

    550-5.7.26 DKIM checks did not pass and SPF check for [storeyconcretecompany.co

    550-5.7.26 m] did not pass with ip"