My Canon MFP is connected via hard-wired Ethernet to a router that is in AP (access point) mode.
The router in AP mode is connected to the main router directly via hard-wired Ethernet cable.
Networking is working as I can ping my printer from a bash shell on macOS or from a shell on Windows 7.
Additionally, I successfully updated the firmware yesterday.
Finally, I can see my printer as active and connected from my router's Network Center Traffic Control screen.
Thus, clearly networking is working.
I consistently get the #753 error when trying to scan to email.
The destination Yahoo email address and password are verified correct. 2-factor authentication is OFF on the account.
The attached screen grabs show the settings on my printer.
I noticed that the MFP resets the Primary DNS Server Address to 192.168.1.1 which is the address of my primary router.
I see many similar posts on this forum, but some are from several years past and not the exact same model printer. Thus I'm posting this with as much detail as I can.
The error would point to an issue with the scanner having problems scanning via ethernet. In this situation, it could be caused by poor network connectivity or a configuration issue between the router and main router.
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Sorry, I wasn't getting notifications of replies to this thread....
I resolved the problem. The problem was that I could not get email to work with Yahoo. I was able to get it to work using a Gmail smtp server.
I did not investigate in detail the Yahoo problem, but I suspect it was related to Yahoo third-party apps and the need to register passwords. I could not figure out how to get Yahoo to recognize my device as an acceptable device.
So I just used gmail smtp servers instead.