This is a highly technical issue, and I can only address the concept. Email servers check your sending email address to the provider of that account to verify that your account is legitimate. For example, if you have a private domain of johndoeA@xxx.com, hosted by hostgator.com, the email server will check to see that your email account is actually at hostgator. My sentence is somewhat generic, but the issue of SPF and DKIM are settings at the host server, not a setting within Thunderbird. I suggest you contact your email provider to address this. The email account works, but is being rejected by gmail, which is very tight on security.