How can I send an email 24 hours after first email?

You can achieve this by many ways. You can use wordpress cron job which has the function like wp_schedule_event(). Go through its documentation and you will get multiple arguments that can be passed through the function.

https://codex.wordpress.org/Function_Reference/wp_schedule_event

Another way is use wordpress set_transient() and get_transient() function. please check their documenation for better understanding.

https://codex.wordpress.org/Function_Reference/set_transient
https://codex.wordpress.org/Function_Reference/get_transient

Also there are many plugins, which can do this job easily.

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