I had this happen with Entourage in El Capitán just this afternoon. It appeared to be a glitch with my ISP (Time Warner, AKA Roadrunner). I resolved it by accessing Roadrunner's webmail and deleting any offending messages from there. My best guess is that there was some corruption in an email that Entourage couldn't handle, but I don't know for certain.
This had happened quite awhile back with an older OS (I don't remember which one) and the cure was the same.
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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