This mail is unauthenticated

[neilmcc]

Hi, having trouble sending emails to gmail accounts from my outlook.com address. It has worked in the past to the same address.
Stupid thing is sending invoices out from Cashbook works.
Are there some easy to understand instructions for me to check why this is happening? I have a look online but got lost on the first accent!!!
Using Outlook 2016 ,I think it is.
Of course, no problem if I use my gmaill account.
Thanks.

Message back to me reads,
     host gmail-smtp-in.l.google.com[74.125.200.27] said:
    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 [outlook.com] did not
    550-5.7.26 pass with ip: [202.137.240.16]. The sender should visit
    550-5.7.26  https://support.google.com/mail/answer/8...entication for
    550 5.7.26 instructions on setting up authentication.
    k71-20020a63844a000000b00553b006cd9fsi3889033pgd.728 - gsmtp (in reply to
    end of DATA command)