Make sure they all use their corresponding SMTP server, not the globally assigned one.

Look under Tools > Accounts Settings > Account name > Outgoing server (SMTP). Click on the default value and m ake sure it's the actual SMTP account assigned to the corresponding email account.

Tried that also, and double checked the setting after your post. All the accounts are using their corresponding SMTP server. No success

Fabián Rodríguez said

Make sure they all use their corresponding SMTP server, not the globally assigned one. Look under Tools > Accounts Settings > Account name > Outgoing server (SMTP). Click on the default value and m ake sure it's the actual SMTP account assigned to the corresponding email account.

Tried that also, and double checked the setting after your post. All the accounts are using their corresponding SMTP server. No success

I read your initial question again and realized there have been changes at your email provider.

I'd refer you to this link, it has extensive information and details, and at the end there are links for both IMAP and POP3 settings. Please double check that and make sure you are following their directions, specifically the information on the username needed (full email address) and exact mail servers.

Sorry, somehow the link didn't make it. Here it is:

https://help.aol.com/articles/verizon-move-to-aol-mail-faq

Not appropriate answer when two verizon emails work fine on thunderbird and the third receives and, after several months of working fine, will no longer send.

Also, the link does not provide "extensive" help.

Also, AOL email support has been shut down by AOL due to "high volume" - interesting eh?

For the sake of reference, I am copying here the information at the link I provide.

Can I use a third party email program such as Outlook or Thunderbird, to access my verizon.net email with AOL Mail?

Yes, you can use a third party email program such as Outlook or Thunderbird to access your verizon.net email with AOL Mail.

If you previously used a third party program to access your mail, you simply need to update your password to the one you created when you registered with AOL Mail. To update your password settings, follow the instructions in the help article titled, Verizon Move to AOL Mail: Updating your third party email program or mobile device with your new account information (POP3).

If you would like to use an email client with your new AOL Mail account and you ARE NOT currently using an email client configured with POP3, follow the steps in the help article titled, Verizon Move to AOL Mail: Setting up your new AOL account in a third party email program or mobile device (IMAP).

Update Verizon.net account info in POP3 email applications Learn how to update your account info for 3rd party email clients.

These steps should be followed only if you've been using a 3rd party email client with POP3 enabled to send and receive Verizon.net email prior to migrating to AOL. If you're not currently using an email client with POP3 enabled, you can set up your Verizon.net AOL Mail account using IMAP.

POP3 lets you access your emails in a 3rd party application. Your messages are downloaded to a single computer or device from a server, then deleted from the server. This means the only copy of your emails are on that specific device.

After migrating your Verizon.net email to AOL Mail, follow the steps below to update your settings based on your email client. Be aware some sections will link to the client's help page, and they can't answer questions about AOL Mail settings, or your Verizon.net username or password.

No matter what application or software you use, the POP sever and port settings will be the same. Just make sure SSL is enabled and you use your full email address, including @verizon.net.

Server Settings Port Settings

Incoming mail server (POP3): pop.verizon.net  POP3-995-SSL
Outgoing mail server (SMTP): smtp.verizon.net 465-SSL

Configure a Verizon.net account using IMAP Learn how to set up your Verizon migrated account in third party email applications using the IMAP protocol.

This article should be followed only if you haven't used POP3 with a 3rd party mail client. If you're a Verizon migrated user and want to continue using your POP3 configured client, you'll need to update your client with POP3 settings.

IMAP is a method used to retrieve emails from a mail server. Your messages are stored on the server, and each time you check your inbox, the email client contacts the server to get your messages. Unlike POP3, your messages aren't downloaded to your computer or device, thus you can sync your account on multiple platforms.

If you switch from POP3 to IMAP, only new messages will move from the server to your devices. Any emails previously saved on your device by a POP3 client, won't transfer.

After migrating your Verizon.net email to AOL Mail, follow the steps below to set up your 3rd party client. Be aware some sections will link off to the client's help page and they won't be able to answer questions about AOL Mail settings, or your Verizon.net username or password.

No matter what application or software you use, the IMAP sever and port settings will be the same. Just make sure SSL is enabled and you use your full email address, including @verizon.net.

Server Settings Port Settings

Incoming mail server (IMAP): imap.aol.com     IMAP-993-SSL
Outgoing mail server (SMTP): smtp.verizon.net SMTP-465-SSL

Doesn't address the issue which is SENDING DOESN'T WORK.