I have heard that bouncing accomplishes nothing for spammers, BUT I'll share this: the software called Mailwasher Pro is FREE if you use it on just one account, and it has the ability to bounce messages and to keep from your inbox if you run Mailwasher prior to starting thunderbird. it may accomplish what you wish. I use the fee-based version as a means to minimize junk in the inbox. On your question, I am not aware of any ability of Thunderbird to bounce messages (but that doesn't mean it can't).

Thank you for your possible solution. I was unhappy to hear that bouncing does nothing to reduce spamming. I guess this is because spammers don't bother pruning their lists, or noting that accounts are missing. Spamming is a real problem with email, and we need better solutions.

I wrote a program several years ago that was supposed to counteract spamming by sending random messages to spammers, with fake sender addresses. It ran all day, keeping the number of outgoing messages fairly low to prevent denial of service problems for recipients. Unfortunately, it had no noticeable effect, and did get my mailserver temporarily listed with one of the standard blacklisting services.

Thank you for your possible solution. I was unhappy to hear that bouncing does nothing to reduce spamming. I guess this is because spammers don't bother pruning their lists, or noting that accounts are missing. Spamming is a real problem with email, and we need better solutions. I wrote a program several years ago that was supposed to counteract spamming by sending random messages to spammers, with fake sender addresses. It ran all day, keeping the number of outgoing messages fairly low to prevent denial of service problems for recipients. Unfortunately, it had no noticeable effect, and did get my mailserver temporarily listed with one of the standard blacklisting services.

I think you have not really thought this out. You send me an email. Within seconds, before Thunderbird has taken down it's sending dialog sometimes, it has been delivered to the recipient domain. Then hours or days later that domain sends out nothing, but a mail server sends an email from a mail client purporting to be from the domain bouncing the email. This whole bouncing thing sounds really good, until you realize the server bounces mail, not a mail client. This process in effect confirms that not only is there an email address (the original delivery was accepted) but someone is actively messing with the spam and therefore probably reading at least some of it. THe perfect person to send more spam to. They are at least looking at it.

Have a read about how legitimate delivery failure notices from spam lists can get folk blacklisted before you embarque on anything so pointless in the future as replying to a spam email. https://en.wikipedia.org/wiki/Backscatter_(email)

You want a better solution. Write to your elected representative and demand change. Australia legislated some 20 years ago on the subject and SPAM almost ceased overnight. The US government around the same time came up with their canspam act and nothing substantive changed. To me the solution is obvious, and it is not technology based.

I agree that a legal solution makes good sense. Not only is the USA CAN-SPAM Act of 2003 too limited to be of much use, it is not enforced at any level of U.S. government, as far as I know.

As to the Wikipedia article, it says, in part: Measures to reduce the problem include avoiding the need for a bounce message by doing most rejections at the initial SMTP connection stage; and for other cases, sending bounce messages only to addresses which can be reliably judged not to have been forged, and in those cases the sender cannot be verified, thus ignoring the message (i.e., dropping it).

I'm wondering how to turn this advice into reality. How should the SMTP dialog be changed to detect and defeat spam email? Existing SMTP libraries already do various checks, but they may not be enough to detect spammers, who frequently create phone email accounts based on compromised email addresses or website pages. And I really don't see how Thunderbird can help, since it apparently does not have any user control over its SMTP dialog during email sending.

Can I, as an end user, make such changes to the SMTP dialog of any email sender? Or does this suggestion require a wider design and implementation?