Edit your account with the new imap settings from AT&T, you could have been using a very old method that was depreciated. So contact them for the current info and you may have to disable your anti-virus as that can cause issues like this as well

Thanks for the information my email problem. Unfortunately it did not solve my problem. My settings were okay? I solved my problem by the following: I went to Thunderbird and created a new email account using the same email address that I was haveing problems with. Idid have to change my name but other than that I now had two emails with the same address. I could send messages with the new account I added. I then went to all of my message folders and added then one by one by name to my newley added account. I then went and copied and moved all of the contents from the folders to my new added account with the same address. It worked! I then deleated the account that was not allowing me to send messages. It took time to select and move all of the messages in the different folders but it is done and it now is working. Thanks for your help!

Thank you for having found the solution. Can you please mark your post as resolved? (and would you join the SUMO team? ;-))

alan49 said

I am unable to send emails on my main email account using Thunderbird? My main email account (@att.net) will not allow me to send emails. I can go to web mail and send but not on the email client Thunderbird. This just recently happened. I have checked all of the settings and as far as I can tell they are correct. I have two other email addressed with att.net and they are working fine. I went to another computer and added this email and it is working fine. But I can’t get it to work on my main windows 7. I have been using this for years with no problem. I would delete this account and reinstall this mail account but I would then lose all of my saved messages stored in my folders. What do I do??? This is the message I get when I try to forward something or send something. “An error occurred while sending mail. The mail server responded: Request failed; Mailbox unavailable. Please verify that your email address is correct in your account settings and try again.” Thanks, Alan

This happened to me. Here's what worked:

Until a few days ago the email address on the Account Settings page for an email address could be different from the User Name under server settings for that same email address. Not any more.

I have two linked email accounts. Email sent to one is forwarded to another and then sent on to me.

I've always shown the FROM address in emails that I send as the second account, not the first one. This meant that I set the User Name under Server Settings as Account 1 and the email address that is displayed to the email recipient under Account Settings was set to Account 2.

That is no longer possible. The displayed FROM address entered on the account settings page now must exactly match the User Name address you show on the Server Settings page.

If there is a mismatch TBird will not send mail.

Make sure that both are the same, then try to send again.