016-722 You should have 3 DNS servers to play with, so make the primary match your gateway, make the 2nd 184.108.40.206 and the third be an actual ISP provided DNS, this way no matter the issue, one will have to respond. You can prove the issue is not in DNS by changing the SMTP server to its current IP address, you certainly won't get a DNS error then.
016-450 Not sure what the "print successful" signifies, the code would have been for a scan to the PC, not the other way around, so being able to print just proves nothing on the printer changed, the PC settings don't matter for that.
The only change made to our network recently was patching a few more ports, too generic to work with, patching implies security, and most have printers on a VLAN that has no access to the outside world, which would explain your errors.
Easiest and most reliable method is to simply do a scan while running a Wireshark trace so you can read the fault as it occurs and know without any doubt what the issue is.
Our ApeosPort-IV C3370 has started failing to scan to email with a confusing set of error codes.
Originally it presented an error message that the IPv6 link local address was already in use. I fixed this by disabling dual stack and using IPv4, as we're still an IPv6-free environment.
The printer then displayed error code 016-450 - "[Cause] The SMB host name already exists". I've verified the host name is set correctly in the admin panel, it's getting assigned the correct IP, and we can print successfully despite the error.
Scanning to email fails with error code 016-722. The manual says this code is for "The staple position specified is not supported", but the scan report says "Completed with an Error (016-722): DNS server un-sets up in SMTP". I've verified SMTP settings are correct.
I've run out of ideas for debugging, as far as I can see everything is configured correctly. The only change made to our network recently was patching a few more ports, DNS, mail etc have not changed.
Any suggestions would be appreciated!