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016-507 is an invalid login, usually due to an invalid response(logs in, but doesn't understand the response and quits), which is what would happen if the server uses a protocol the printer doesn't support. If that is the case,try a firmware update to get whatever update may be needed, else SMTP2Go should work.
Of course you may have just changed the password for the account the printer is using, or enabled 2 factor authentication or something on the account, but if nobody changed anything, it will have to be a server side change that maybe updating the firmware will fix