Relaying is when you send an email via an smtp (outbound) mail server other than the one which belongs to your ISP.
suppose my ISP is called fe.com
the smtp mail server might be:- mail.fe.com
However, I have a mailbox on my own domain name, which is:- forum.com and I have configured Outlook to send all my outbound mail via the smtp server called mail.forum.com even though I'm connected to fe.com
the fe.com mailserver might see my mail as spam,and think that I'm trying to relay messages through it to hide the point of origin, so it rejects the mail and bounces it back to me with a relaying error message.
Check your Outlook account settings and make sure that your outbound (smtp) server is set to that of your ISP.