What does your mail provider have to say about why mail from your account is not being delivered? Usually if the send is refused a report is made to the sending server. They have access to logs with this information. No one here does as Thunderbird is a client application that only gets what the provider tells it.

As you have been through all the whys stuff you said, I will not bother to repeat it..

The thing is, I can send and deliver emails from all other devices BUT my laptop. So it seems it must be a security setting but I cannot figure out what. I have turned off the firewall, turned it on, rebooted, etc., and nothing works.

Got a signature you use on your laptop? they are by far the most common cause of deliverability issues. They usually use images for socail media that gets them flagged as spam.

I am not sure what you mean by a "signature" - could you please clarify?

If emails are being sent, but fail to be received, you may receive a mailor daemon email stating it could not be delivered and usually you can work out why. eg: person you sent email to has a full Inbox and therefore cannot receive anything or perhaps the email address does not exist or it is blocked due to potential spam.

Servers are more savvy about spam these days and can simply block the email from getting to final recipient or put in recipients spam folder.

If email has too many links it might be thought to be spam. It has been known that links etc in signatures can fil some spam filters. Some people use a signature in their emails. eg: regards John Doe

This is not something they type each time, it has been auto setup to be included. Some people make their signature a bit more fancy and include an image or clickable link to their website etc. Many create and use a html file to add their signature.

Signatures are set up within Account settings. see image below.

Some servers can get funny about emails that are only sent as html, so are you sending as both plain text and HTML? 'Menu icon (3 lines)' > 'Options' > 'Options' > 'composition' > 'General' tab click on 'Send Options' select 'Send the message in both plain text and HTML' click on OK

see image as guide.

Are you using a POP or IMAP mail account ? If IMAP and you send an email, do you see that email stored in 'Sent' folder in webmail account - if you logon to view your webmail account via a browser?

Send a test email to me to the following email address, note I have written it phonetically deliberately, where I say 'at' it means '@'. email : mail at anje dot co dot uk