During our diagnostics we attempt to simulate sending a message to a fake email address; firstname.lastname@example.org. We do this to try to detect if your server is an open relay, which means that it accepts mail to domains for which it is not responsible and then passes it along to the proper server. Your server responded with a 200 accepted code to our RCPT TO command. THIS DOES NOT MEAN YOU ARE OPERATING AN OPEN RELAY, only that you may be an open relay.
Many servers now pretend to accept incorrectly addressed email but then discard those message without forwarding them. This technique is used to prevent a directory harvest attack. In a directory harvest attack a spammer will attempt to send to thousands of email address at your domain trying to find real ones. When your server responds with an error (5xx), the spammer knows that it is not a real email address. When your server accepts the message (2xx), the spammer knows that was a real email address.
Are you managing your email delivery?
Email delivery is more than running an email server, or regularly checking for blacklists. To really ensure you customers get your email, you need to setup SPF, DKIM and DMARC. If you aren't manage your DMARC configuration and responses from email providers, you don't know if your customers are getting your email.
MxToolbox is YOUR expert on email deliverability. MxDelivery Center can help you setup and analyze your DMARC, DKIM and SPF to give you the insight you need to make email configuration changes and get your emails to your customer's.