This seems similar to a problem that I had (not with Yahoo, however). See this thread for details. A harmless thing to do is to set up a new account in Mail with the same password. In my case, apparently my account had become corrupt. Whatever you do, DO NOT DELETE THE OLD ACCOUNT. You'll see the reason in that thread.
Jon
macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365
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