I mean no offense, but this is like attempting to hammer a nail with a screwdriver. Thunderbird was not designed for such. There are several low-cost or free email services available that will remove you from those concerns of spam and unsubscribing. They can even give you feedback on who is, and is not, reading your messages. I have been there and I seriously encourage your investigating such services. For example, Mailchimp will let you set up a free account, provided that you send no more than 1,000 messages per month (100 subscribers times 10). So, if you send fewer than 10 messages a month, you're in for free. And there are other similar email providers. Good luck. :)

Thanks for your reply david. I've used a free Mailchimp account for years, but recently, they've applied more and more restrictions - forcing you to buy an account. Hence, I canceled my account. Lately, I've used a free Brevo account, but I can't personalize my emails like I can with Thunderbird and the mail-merge add-on. I access spreadsheets to report handicaps, points, attendance, and other stats, and the guys love it. When I send my email to a test site (like mail-tester.com), I pass all of the tests, except for the unsubscribe link they want to see in a header - "Your message does not contain a List-Unsubscribe header". Any ideas? Thanks again.

In that case, you could create an HTML signature with a response option. But that requires that they read in HTML format. I mention as a sig because every mailing list I'm on has the unsubscribe in the footer of the message.

You can probably add custom headers to composed messages by following this article:

https://kb.mozillazine.org/Custom_headers

Thanks sfhowes. I'll look into that.

I was just about to share that same https://kb.mozillazine.org/Custom_headers article.

PS: @firefoxManager - You wouldn't happen to be the same person who recently wrote a nice note to thunderbird staff, would you?

Yes Thundamike. All was going smoothly, and then it wasn't. Suddenly, my newsletters were going to the guys' spam folders, and I was getting nasty messages stating that spam was suspected in my emails. It's been a rocky road ever since. I've started using mail-tester.com to try and find out what I'm doing wrong. I get the following results: "The List-Unsubscribe header is required if you send mass emails, it enables the user to easily unsubscribe from your mailing list. Your message does not contain a List-Unsubscribe header" Hence, I'm asking how do to that.

Sorry to hear you have been having issues. Everyone at the company who saw your kind note really enjoyed reading it. Thank you for that.

As to how to fix your issue: It's not easy out of the box. But maybe we could go Some services like Sendinblue (Brevo), MailerSend, or Postmark let you send emails via SMTP and allow you to include custom headers like List-Unsubscribe. You’d still be using Thunderbird + your mail merge setup, but your emails would go through their servers and you can often set unsubscribe headers in your account settings or templates.

Well clearly, you folks do good work! With the article you provided, I did manage to add a list-unsubscribe header to a template within Thunderbird, sent a note to mail-tester, and it passed with flying colors. But a local email service, to which many of my guys subscribe, still wouldn't deliver the mail. I may have to wait them out. I do use Brevo as a backup, but I don't see where I can still use Thunderbird + mail merge to send emails to their servers. I'll investigate the other services you listed. I do appreciate your help.

Fantastic news! Glad to hear that the majority of your golf group is back on track.

I will mark this question as solved for now. We can always reopen or file a new ticket if we need to dive in further.