Make sure the security setting in Yahoo for "Allow apps that use less secure sign in" is enabled.
More at: https://forums.yahoo.net/t5/Sending-and-receiving/Pop-outlook-2016-Yahoo-mail-Plus/td-p/3522/page/3
<p>Make sure the security setting in Yahoo for "Allow apps that use less secure sign in" is enabled.</p><p><span style="font-size: 10pt;">More at:&nbsp;</span>https://forums.yahoo.net/t5/Sending-and-receiving/Pop-outlook-2016-Yahoo-mail-Plus/td-p/3522/page/3</p><p><span style="font-size: 10pt;">&nbsp;</span></p><p>&nbsp;</p>