Hey there and welcome to Apple Support Communities MeredithLeJeune,
We can certainly understand your concern and are here to help.
We would like to ask you a couple of questions in order to get a better understanding. First, have you tried restarting the device? Restarting while simple, can help resolve most minor issues. Also, have you tested a send out later email 5 minutes from now? If so, does those emails go through? Second, have you tried refreshing mail? Finally, have you tried booting into safe mode? If so, does the same issue happen there?
While you gather that information, check out this link on how to send scheduled emails: Write, send, and schedule emails in Mail on Mac - Apple Support
We thank you for your answers, they will help guide us to the next troubleshooting step.
Kindest regards!