This is generally the issue when the receiving mail supporter (in this case, Gmail) cannot confirm that the sending domain matches the domain of the host sending site. For example, if your business is cakes.com and your business hosting provider is bluehost.com, Gmail sees that the actual host domain is bluehost and looks for confirmation that the settings for cakes.com are reflected there. This ties to DKIM and SPF settings on the server and are beyond anything TB can do. My suggestion is to check with your hosting provider.

Hi David, Thank you for your help. Thunderbird does not offer any support so I am wondering if we need to find a new place to use our company email as it is just an [email protected] email address. I am assuming our "host" is Thunderbird? Sorry I am somewhat knowledgeable but not enough apparently haha. I am also not sure about DKIM or SPF settings. But if Thunderbird is considered our host is there another one that is free to use that you would suggest for us to be using instead? We have never had this problem until this week and have been using it for quite some time now. If you are unable to provide anymore suggestions, thank you again for what you have given us!

Someone in your company is paying someone for the use of that domain name and for providing an email account. That person can direct you to getting this resolved. TB is not a host. All TB does is retrieve mail from the account and send messages via the account. There is no need to change from TB, as it is working properly. DKIM and SPF are technical terms that you (and I) don't need to know and are used by the web hosting company. This is probably a simple fix, once you notify them.