@Netbug 

I find that has a related solution and the walk through instrucion on this link, https://www.tp-link.com/us/support/faq/2919/

@John1234 Thanks for the suggestion, unfortuantely, I have tried all of these as mentioned in the original message, to no avail. Support recommended I try smtp relay service from gmail, however, that is only available for Google Workspace accounts.

There is no reason that I can see why Gmail nor Yahoo! Mail authorized app passwords would not work as they work in all other instances that I have tried. And it's IMMENSLY frustrating that the addition of Cloud Administrators has to be done through the email service on the device/controller rather than through the TP-Link Omada systems notification service.

@Netbug Have you actually logged in to the gmail account and used it to send and receive an email normally? As it is a new account they may be blocking other use of it until used.

@JP77 Yes, I initially tried it with my personal account. No go. Then I tried a Yahoo! account. No go. Then I set up a new Gmail account. No go.

As pretty as the Omada system is, it's not very clear on it's errors. Like, it just says the email failed. It doesn't say why it failed. And it's coming back really quickly, so it doesn't feel like an authentication error; it feels like it's not reaching the server. I wish it would tell me if it couldn't reach the server, if authentication failed, if the server stopped responding, etc.

@Netbug Have you tried sending the test email to a different address other than the sending address?

@JP77 Yes, I have. I tried my personal gmail account, my yahoo account, and my brother's gmail account. All are generating the failed message. :(

@Netbug Hello,

working this setting:

SMTP: aspmx.l.google.com

port: 25 (without SSL)

authentication: yes

sent test email to - must be any gmail.com

@PavelV Thanks very much. Unfortunately, same error. Literally the exact settings you posted. Tested with both the app password and the regular password. Something very strange going on here.

@Netbug After test I received email from gmail "A less secure application has been blocked" In account is necessary allow this less secure applications.