Version 57.0a1 (Aug 28 version) doesn't download Yahoo mail messages (even message titles) at all in IMAP. Somehow folders are downloaded. This happens current release version. Only sending is possible.
Maybe related to bug 1394642. We haven't made changes to IMAP recently. Do other version of TB work? Like TB 52.3 ESR, TB 56 beta or an earlier Daily?
Until some days ago, Daily had been working properly with Yahoo mail. I'm using IMAP with gmail, but they are normally working. Only Yahoo is affected however many times I deleted the account and add the account.
It's not a TB problem, maybe https://support.mozilla.org/en-US/questions/1172099 is related.
When you suspect something broke in a nightly build, the definitive test is to download and try the last "older" version that you remember working. http://archive.mozilla.org/pub/thunderbird/nightly/2017/ Then, if it is a Thunderbird bug and you have a one day regression range, we have something solid to find the cause.
I'm sure Aug-1 build (56.0a1) was working, but if I tried it now, it doesn't work either. It may be a Yahoo problem.
But I must mention Outlook 2016 is working with Yahoo Mail perfectly now.
oh, now Aug-1 build (56.0a1) is working with my Yahoo account.
After reinstalling Aug-29 build (57.0a1 64bit Windows), Yahoo mail somehow is working properly. Very weird.
No, it's not weird. Yahoo are changing things while you're trying different versions.
I got it. Then it's invalid.
https://support.mozilla.org/en-US/questions/1168153?page=2#answer-1001386 confirms yahoo having issues
OMG! It's a paid account! Thank you for investigating!
Created attachment 8902871 [details] tb-yahooerr.png When Yahoo mail fails to download, this error message appears.
Thanks for letting us know.