016-722 You should have 3 DNS servers to play with, so make the primary match your gateway, make the 2nd 8.8.8.8 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.