Suddenly unable to send or receive email messages

I use Mozilla Thunderbird as my e-mail client for an atlantic.net server and I have trouble the past few days both sending and receiving e-mails. I have no issues with the Atlantic.net web client, only my Thunderbird client. I have attached screen shots of both errors appearing after I launch Thunderbird and after I try to send an e-mail. I have also attached my server settings. I have made no changes in server settings. These issues began to appear a few days ago after running flawlessly for several years. I contacted atlentic.net and opened a ticket with them but they say it's an issue with Thunderbird.