550 validating sender failed to meet spf requirements
If one of these spam messages can’t be delivered, an email server might send an NDR like the one you got, back to the From address.Such an NDR is useless rubbish because it creates the false impression that you did something wrong.To do this, open the mailbox in a web browser that supports only the light version of Outlook Web App, and use the following procedure: Remove all recipients Solution 4: Update forwarding rules to remove incorrect addresses Sometimes the NDR is caused by a recipient that the user didn't directly send mail to.It could be that the intended recipient has either a forwarding rule or a delegate set up for their mailbox.Check your Sent folder for messages that you know you didn’t send.
If not, it’s possible that a spammer or someone else inappropriately used your email address when they sent the message.
You can verify this by comparing the address provided in the NDR with the address shown in the original message that is in your Sent folder.
If the addresses don't match, contact the person you sent the original message to and ask them if they have an email rule to forward messages and ask if they know it's working correctly.
Check your Sent folder for any messages that you know you didn’t send.
If you find any, it’s possible your email account was compromised.