Simple answer: do not use the email address when sending or replying or forwarding emails. Sending is not automatic, you have to generate the email and click on a Send button.

You could use an smtp server that will not allow sending using the incorrect server for the email address, so that it cannot send, but if you accidentally use the email address you may get an error message, but at least the email will not be sent. Do this here:

Tools > Account Settings for the mail account. bottom right Outgoing Server - select one that will not accept sending using wrong email address

OR in Outgoing Server(SMTP) select the server you are using for that account click on 'Edit' deliberately make an error in the server name - remove the port details etc. So the details are wrong and cannot send.

Thanks for the info. I just put "FAIL." in front of the smtp server for the accounts I want to prevent from sending email, and it works.