Yesterday 8/15/15 Thunderbird performed an automatic update when I opened it. Since then we can't send email from my wife's account, [email protected]. I tried sending from other accounts and the system worked OK. I checked the settings as suggested by the error message, and all seems ok. An error occurred while sending mail. The mail server responded: 5.1.8 [F18] Sender [email protected] mailbox is unavailable.. Please verify that your email address is correct in your account settings and try again This is the first problem we've experienced since we started using Thunderbird a couple of years ago. Thanks. Bill Darbyshire edited emails from public and spambots.