Does the address have a leading or trailing space or uncommon characters? Spaces sometimes are the result of improper importing of contacts.

I have several accounts and send out on any of them. However, I have one Gmail account and it sends without problems. The others are ATT/Yahoo related. I contacted ATT, there isn't a problem on their side. I can send out mail if I go directly to the account.

I meant to say, "I have several accounts and (I can't send) out on any of them."

Sending from a webmail portal is different than sending over smtp in TB. First, make sure each account is set to send on the smtp server for that account, not the 'Default'; Tools/Account Settings, select the account in the left pane, then look at Outgoing Server (SMTP) in the lower right pane.

What are your smtp settings for the ATT account? Their recommendations depend somewhat on the email address

No sorry, I am still getting this message when I try to send out email.

"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."

Go to Help/Troubleshooting Information, click 'Copy text to clipboard', then paste it into a reply here, omitting all the printer and font data.

Application Basics Name Thunderbird Version 52.6.0 User Agent Mozilla/5.0 (Windows NT 6.3; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0 Profile Folder (Local drive) Application Build ID 20180123185941 Enabled Plugins about:plugins Build Configuration about:buildconfig Memory Use about:memory Profiles about:profiles Mail and News Accounts ID Incoming server Outgoing servers Name Connection security Authentication method Name Connection security Authentication method Default? account1 (pop3) inbound.att.net:995 SSL passwordCleartext smtp.mail.yahoo.com:465 SSL passwordCleartext true account2 (none) Local Folders plain passwordCleartext account3 (pop3) inbound.att.net:995 SSL passwordCleartext smtp.mail.yahoo.com:465 SSL passwordCleartext true account11 (pop3) inbound.att.net:995 SSL passwordCleartext smtp.mail.yahoo.com:465 SSL passwordCleartext true account12 (pop3) inbound.att.net:995 SSL passwordCleartext smtp.mail.yahoo.com:465 SSL passwordCleartext true account17 (pop3) inbound.att.net:995 SSL passwordCleartext smtp.mail.yahoo.com:465 SSL passwordCleartext true account18 (pop3) inbound.att.net:995 SSL passwordCleartext smtp.mail.yahoo.com:465 SSL passwordCleartext true account19 (pop3) inbound.att.net:995 SSL passwordCleartext smtp.mail.yahoo.com:465 SSL passwordCleartext true account20 (pop3) inbound.att.net:995 SSL passwordCleartext smtp.mail.yahoo.com:465 SSL passwordCleartext true account21 (pop3) inbound.att.net:995 SSL passwordCleartext smtp.mail.yahoo.com:465 SSL passwordCleartext true account22 (rss) Feeds plain passwordCleartext account23 (imap) imap.gmail.com:993 SSL OAuth2 smtp.gmail.com:465 SSL OAuth2 true

Subject: sfhowes posted an answer to your question "Sending email error instructs to verify email address. Address is correct and I still can't send out emails." Date: Fri, 9 Feb 2018 00:58:19 +0000 From: Mozilla Support Forum <no-reply@support.mozilla.org> Reply-To: no-reply@mozilla.org To: teresawilde@bellsouth.net

Mozilla Support

For bellsouth.net customers, they recommend using outbound.att.net on port 465, SSL/TLS security, authentication = normal password, and User name = CustomerID@bellsouth.net. That is for POP accounts with inbound.att.net on port 995 (other settings same as smtp).

On that page, there is a Community Discussion post that confirms those settings. I suggest you change the settings for one account and see if it works, then change the others if it's successful. Tools/Account Settings, select Outgoing Server (SMTP) at the bottom of the left pane, then Edit the appropriate smtp server. You may need to delete the existing smtp password from Tools/Options/Security/Passwords/Saved Passwords; you will be prompted for the correct password when TB connects to the smtp server.