This has nothing to do with Thunderbird. Gmail thinks you are sending out spam for some unknown reason and is blocking those emails. They have given you a link as information. https://support.google.com/mail/answer/69585 At that link, it specifically tells you what you can do, it offers more info on guidlines and offers a 'contact us' link. there are other links on that page such as: https://support.google.com/a/answer/168383?hl=en There is no point in me copy pasting the same info, so follow the link gmail sent you and read the information. check your email Subject and content so that you are not using commonly used spam words. If you are regularly sending one email to a group of people, there is a function called 'Mailing lists' in TB. If you have not come across this, here is some info. * https://support.mozilla.org/en-US/kb/how-create-and-use-mailing-lists-thunderbird It would seem this has happened to several people in the past such as: * https://productforums.google.com/forum/#!topic/gmail/sEJFjhJ5BD8 ''yaramayer said:'' ''The only way I have found to fix this each time is to access the problematic account's webmail, clear the login captcha if it appears, and/or go to the account UnlockCaptcha page'' I've also located this information which may be of help: * https://support.google.com/mail/answer/81126?hl=en