If it is Thunderbird tagging it as Junk, mark it Not Junk. You would have had to enable Thunderbird Junk controls in Server Settings for it to work in the first place. If it is your provider or your antivirus software making this determination you need to pursue those avenues to solve it.

As Airmail says, check your junk Controls. Do it for each mail account. Right click on mail account in Folder pane and select 'Settings' to open Account Settings.

To access Options: Menu icon > Options > Options > Security > 'Junk' tab

See images as guide. You can set up address books as whitelists.

More info: https://support.mozilla.org/en-US/kb/thunderbird-and-junk-spam-messages

I do not have an email to work with. This was a message from the company's web site that just said to add "customers.xxxx.com" to my safe list. I do not have the prefix for their email account. I guess I'll have to ask them to send me an email just to get to mark it as Not Junk..

Thanks anyway...

re :This was a message from the company's web site that just said to add "customers.xxxx.com" to my safe list.

If you did not have an email then you must have read it on the website asking you allow a specific website to a safe list. It sounds more like they are trying to get you allow it in a browser. But that has nothing to do with an email client.

From an email point of view, they may be asking you to allow remote content from that company in any email they try to send to you. I'm not so sure you want to agree to that.