In cases where you try to send an email to Digitalchalk Support at email@example.com, it could be that the sending email IP address has been blacklisted as spam. This could be a result of a compromised email account, an Open Relay, or simply sending mass marketing emails.
You can check that by getting the IP address from the undeliverable email you get after sending the email to firstname.lastname@example.org This will normally look something like "Received: from your.domain (your.domain. [IP address]"
You can then search MX toolbox with the IP address to see if your IP has been blacklisted. Then you can get suggestions on how to get it removed from that blacklist by clicking on "details" in the reason column. Please note that you would need to create a free account on MX toolbox to get the details.