016-503 means the SMTP server hostname is wrong/can't be reached/timed out.
Assuming that the mail server info has not changed and nobody messed with the configuration in CWIS, then your issue would have to be the DNS settings on the 6015. To test it, set the primary DNS to 8.8.8.8 and
My WC6015 suddenly stopped working on scan to email and provide error code 016-503. There are many things that are happending, like the ethernet port does not seem to work. I have tried to upgrade firmware but it does not seem to be going to the printer from the computer (Ihave tried the network as well as the usb options - the firmware upload seems to go through but the printer does not respond). Back to the scanning issue - I have reset the email server settings without luck.