That isn't Thunderbird's strengths, but this URL may help:

https://www.saleshandy.com/blog/gmail-blocking-emails/

Hi David,

Thank you for the reply but we cant really go back to our customers telling them its their fault. Many of them are not technically minded either and just wont understand what we are saying. Seeing as it happens with all Gmail accounts, I assume its something we are doing wrong anyway. They just do not receive our email, and we get nothing back to say its not been sent or delivered. Its very confusing and frustrating. Thank you for your help.

My suggestion was that the URL I shared may help you find out what you are doing that is causing the problem. It may not be anything you are doing; possibly, you are sharing a web server with the same IP address as a spammer. there are many websites with information on how to remove a block; writing to Thunderbird is not the way. Thunderbird is just your messenger. You need to actively pursue this, as other domains may also block you. Contacting Google for support is also a possibility. Good luck.

Hi David, thank you for that reply. That is really interesting and helpful. I had assumed it was a setting on Thunderbird, but clearly not. Thanks again.

There might be something in the format of your messages that is triggering the gmail spam filters, but not those of other services. Besides the factors mentioned in the blog link, try sending a plain text message with no signature or attachments or links. If it's still not received by gmail users, the blocking is probably related to the authentication of your domain, not anything to do with message format.