A colleague attempted to collect his client’s AT&T Yahoo inbox and ran into something called AT&T Active Armor. Apparently this precludes using an email client or app. FEC has bailed me out of unusual email collection issues in the past and I wanted to see if Arman has run into this. FYI, I am not currently a subscriber, I haven’t had an email collection project in well over a year.
ActiveArmor sounds similar to the security add-ons ISPs offer to their customers. You can find some details here:
My guess is that your colleague ran into the recent Yahoo limitations such as the inability to generate an App Password or item caps in folders. FEC is able to work around both with modern authentication, and Yahoo/AOL folder cap bypass respectively.