This is not really a Thunderbird problem, so you should be looking for information that is all about emailing and marketing However, I hope this info may be of assistance. There may be several reasons - a few below, but read the info at the links I found, as they explain more. The recipient may have instructed their server filter or email client filter to send any emails from your email address to Junk or Trash. The recipient may have instructed their server filter or email client filter to send any emails with specific Subject content to go to Junk or Trash and unfortunately you happen to be using common words that are detected as possible Junk/spam. The receiving server or a spam program like spamassassin is detecting any emails with specific Subject content to go to Junk or Trash and unfortunately you happen to be using common words that are detected as possible Junk/spam. I suggest you read some info such as these links: * http://emailmarketing.comm100.com/email-marketing-ebook/flagged-as-spam.aspx * http://blog.hubspot.com/blog/tabid/6307/bid/30594/A-Marketer-s-Guide-to-Getting-Past-Email-Spam-Filters.aspx * https://documentation.mailgun.com/best_practices.html#email-content