I suddenly could not receive emails on my Macbook with Thunderbird. It continued to work find on my Windows computers with the same account settings. After exhaustive trouble shooting I discovered that if I turn off STARTTTLS and set Connection Security to None and Password, transmitted insecurely in my account settings, Thunderbird works normally on the Mac. On the Windows computers, Connection Security set to STARTTTLS and Normal Password works just fine. It is only on the Mac that the issue exists and only in the last several days. Before that it worked fine with the same settings as on the Windows computers.

Sending has always worked fine on the Mac. I just cannot download emails, save emails to the Sent box, or save emails to Drafts with the STARTTTLS activated.