That is really their problem and not yours, in the sense that there's little you can do about it.

Some users have reported that messages sent using Thunderbird get put into Spam folders, whereas messages sent using the email provider's webmail (through their browser) is successful.

I would interpret that as meaning that the provider's smtp server or their user IP address range has been blacklisted, but their webmail server hasn't. And I suspect that webmail is not an attractive vehicle for spammers, so they don't use it and so it doesn't find its way onto blacklists.

If a helpful would-be recipient could return to you a message that was put into spam, you could use it as evidence in a request to your email provider to work with the blacklist operators to remove your provider's servers form the blacklist.

A quick and dirty work around might be to use webmail for now.

Someone said its because I have attachments in my signature :(

Its losing me bookings and work

I would suggest replying on plain text... that rarely gets the attention of spam filters, be very selective as to images in your signature, include them in the message rather than as remote images.

In Thunderbird remove the reply to in your account. Right click in the folder pane, select settings and clear the reply-To: Some servers check that and spam anything that has it, or anything that has it and it is different to the From address. With it set to blank replies will go to the from address anyway.

Other things are. If you have your own domain, check it out here http://www.dnsstuff.com/ (checks your domain is correctly set up on the hosting service.

Check your email address/domain here https://www.spamhaus.org/ (one of the more popular spam block lists)