Please share more information. Thunderbird does not do that, so, with more information, possibly someone here can assist you. For example, I use TB with Gmail and have never experienced what you described. Thank you.

Thank you for trying.

Lets start with the incorrect assumptions.

1. Bulk mail is an invention of Yahoo inc. They use the term for their spam folder in IMAP accounts. Interestingly they label is spam in their web mail. 2. Thunderbird runs on your local computer and connects to your mail providers server to collect mail from them. It is not connected to a server under the control of anyone involved with the Thunderbird project, except to ask if there are updates to the software or to report telemetry information.

That you are seeing a bulk mail folder indicates that you are using the IMAP mail protocol as it only exists in Yahoo mail IMAP accounts on Yahoo mail servers and is replicated to Thunderbird because it exists on the yahoo mail server.

If you have the adaptive junk filter disabled then this is most emphatically not a Thunderbird issue. I have seen mail placed incorrectly in spam folders when anti virus products get involved in classification, but the vast majority of issues are because yahoo have their own junk/spam filtering. It is not very good iun my opinion and has no way to opt out.

I suggest you ask Yahoo for a solution to your problem with their mail servers.