[xyzzy]

I have always thought that the POP/IMAP/SMTP protocol does not include any identification about which email client is being used. When I look at the exchange log in my Thunderbird I don't see anything that says "I am Thunderbird". Well at least I didn't notice anything.

But some services seem to know which client is being used. For example, I've read that Thunderbird won't work on Yahoo with OAuth2 authorization since yahoo considers Thunderbird a less secure app. How could yahoo know that Thunderbird is being used if the protocols didn't identify the app?

Just curious because maybe I've been wrong about the identification all these years thinking there is no need for the servers to know which client so long as the proper protocols were followed.