There's an explanation here: https://www.penpublishing.com/support/emailerrors/understandingemailerrors/ You'll see that the error message comes from the mail server, not from Thunderbird itself -- so to understand why it's happening you'll need to talk to the company that provides your mail. As a first step, the next time that you get this message, try to log in to your mail account via the company's website. If you can do that, and if the mail account is working OK, then contact their helpdesk and ask what's going on. Some email providers these days want to 'encourage' (force?) their customers to use webmail rather than email clients like Thunderbird, because it's easier for them to maintain the webmail interface than to support remote delivery. So they may be restricting the number of connections from remote clients or there may be some other problem with that.