Configure Thunderbird to send messages at a later time
If you use distribution lists within Thunderbird and you add a distribution list to a message and then Send Later, the list will be expanded when you schedule it, not when it is sent. Any changes you make to the list between when you schedule the message and when it is sent will not be reflected in the sent message.
Attachments are frozen when messages are scheduled to be sent. In other words, if you attach a file to a message, then schedule the message with Send Later, then modify the file on disk before the scheduled message is sent, your modifications will not be included in the sent message.
This is also true with recurring messages — when a recurring message is sent and automatically rescheduled, Send Later does not capture a new version of the attached files.