It is probably the cox SMTP server has a reputation problem. They don't use that server when they send from the webmail. One of the reasons everyone has so much trouble with ISP s8upport over email issues.

The secondary issue is usually web mail does a poor job, or none with mail signatures, spo mail sent from webmail usually does not include them. Often the images and other details contained in signatures trigger spam tools. This is especially so when the signature includes those little images folk "find" on the internet for social media links. Frequently they have been used in spam emails and the providers remember those things and add them to their spam filters,

I did have a signature on my email.

I have removed it and still have the same problem.

I have worked with COX twice. If I send directly from their email site, there is no SPAM notation. It's only when I use Thunderbird.

Is your account IMAP?

Please send me an email... Send two, one to devilsgatedrive at gmail dot com and one to matt_au at gmx dot com. Just a copy of something you previously sent that was marked as spam would be good. Perhaps I can figure it, perhaps not.

Just select the mail from your sent folder and then edit as new (ctrl+E) to address it.