what is it with obscure software today. I had never heard of postmaster tools till your post.

Are you high volume business using Thunderbird to send hundred or thousands of emails a day? That is what those tools look to be designed for.

Are you a single user with mail going to spam? Then you probably need to look at your emails and most notably your email signature. Mostly when folk find a significant percentage of their emails going to spam, it is spam. often that company advertising and linked logos to a raft of social media accounts that send the spam tools into a spin.

I m having google group and i send mails to my group members with my group webpage link some times . my group have around 50k members . Many peoples complain about mails going to spam and i have a template to send mails . recently i heard about google postmaster tools . but it allows only private domains to get added with it like funnest.com . Post master tools add mails with SPF and DKIM . is it possible to add such keys in the mails which i sent from thunderbird of my gmail accounts linked in it .

if you are sending 50,000 mails then you are going to have huge issues sending mail with any mail client.

Try using a professional mailer such as mailchimp. https://mailchimp.com/pricing/ AS you usage basically exceeds what could be called "startup" volumes I doubt you will find any free packages. Most "Social" clubs, church groups etc fall into free accounts as they only send a few thousand mails a month.