Thunderbird has already a nice tool to put emails in a waiting email folders.
But when we decide to send the emails (menu file -> send waiting emails) all the waiting emails are sent immediatly one after the other without any delay between them. This causes some problems with smtp servers.
A nice improvement would be to be able to parameter a delay between mails (by default 5 seconds would be nice for instance) in order to avoid any traffic jam for smtp servers.