I have been wrestling with this for months, if not years... Thunderbird (now on 60.8.0 and earlier releases) will not reliasbly always save copies quickly to sent foldder. Sometimes it is instantaneous, other times it just hangs and hangs. When it does I have to click the try again method TWICE... My ISP consultants have double-checked the installation and the email servers their end; I have changed the Sent folder setting to Other and then specified sent folder. I have uninstalled and reinstalled Thunderbird. Any ideas on anything I have not yet tried?