someone somewhere tried to put your domain name as the “From” on unauthorized e-mails (spam) and the mail server they tried to send it through is reporting it to you, that’s all it is. it happens to everybody and there’s not really anything that can be done about it. assuming your DMARC policy is “p=reject” then the e-mails are being rejected and the system is working as intended. but if you’re not using a “reject” policy then the spam e-mails might actually be making it through and you should consider tightening up your policy.
the attachment should just be XML data, likely gzipped, although the contents are unlikely to be very interesting