The problem was the two-step verification process with iCloud. Basically, if you want to use Thunderbird and iCloud together you have to generate an app specific password when you log into the Apple ID website. Here's a link that has the instructions.
Using a Custom SMTP Server - Power Diary Knowledge Base Jul 10, 2020 ios - Iphone: outgoing email iCloud account and gmail The iCloud sender email address will however still be the iCloud address. If you use the build in Mail.app, you can manually choose your Gmail address (and the mail will appear in Gmail's sent folder), but how you can use this one, the way you want to use it - I don't know.
Feb 27, 2020 · Use iCloud Preferences on your Mac in OS X Lion 10.7.4 or later. Use iCloud for Windows on your PC in Microsoft Windows with Outlook 2010 through Outlook 2016. Set up two-factor authentication and, if needed, generate an app-specific password* to use for iCloud Mail. Use iCloud settings on your iPhone, iPad, or iPod touch with iOS 7 or later.
Setting iCloud email address on Outlook 2016 - Microsoft Apr 27, 2017 Find your SMTP server address - smtp mail server
SMTP.gmail.com ip address list. - Google Support
Best alternative to Gmail and iCloud for email, calendars in1-smtp.messagingengine.com; in2-smtp.messagingengine.com; If you want to support email addresses that use a subdomain of the domain you’re adding, add additional entries with a wildcard. I use Cloudflare for my DNS servers, and this is what my DNS records look like. FastMail MX records in Cloudflare