I sometimes go to send an email with Thunderbird and I get the following message (see also attached image): An error occured while sending mail. The mail server responded: Requested action not taken: mailbox unavailable Failure sending mail. Try again later. Please check the message recipient “[email protected]” and try again. [OK] If I go direction to my mail.com and try sending an email, it sends. If I save the email in my Drafts folder and wait for several hours or until the next day, it then will send fine.