The Yahoo window opens and I enter my email address and password and get a Yahoo error page that says “Uh oh, there’s been a problem, try again later.
Try completely clearing the cache in your browser (All time) or failing that try setting a different browser as the default & see if that works.
Is the secure mail key the same as Yahoo app password?.
From Googling that question I believe they are separate things.l as @Proteus advised.
Also in my view “you shouldn’t need an app password at all with eM Client” with the Yahoo account as the mail, contacts, calendar etc are all added into eM Client using the OAuth token (like it does in my Outlook Yahoo setup) example when you click allow, and the generated app passwords are normally “only for less secure apps”.
Fom adding my Yahoo account into eM Client V10 “via the automatic account wizard” around 50 times to test, “one time after I completed the setup” for a split second I saw the Contacts and Calendar appear and the online data was there in each section of eM Client, but then they both dissapeared in front of my eyes while watching them and never reappeared.
Now it’s not the OAuth token part issue as that allways adds into the Yahoo online end, but seems to be the part (after that) which adds in the Yahoo Contacts and Calendar to eM Client.