This could be caused by Google not recognizing and blocking Thunderbird's default user agent; try toggling general.useragent.compatMode.firefox to true (which basically has TB emulate Firefox's user agent)
74 karma · joined August 28, 2015
> An incorrect login says so without presenting a captcha.
Both of Microsoft's own identity services (AAD and Live ID, and by extension O365 and Outlook.com) recognize (and allow creation of) microcolonel@example.com, micro.colonel@example.com, and mic.rocolonel@example.com as distinct identities/email addresses.
What you're saying is that once the first of (microcolonel|micro.colonel|mic.rocolonel)@example.com registers at github, the other two will no longer be able to do so, but will instead receive a confusing 'you already have an account' error, (hopefully) without being able to receive password reset emails.