Email Scheduling

Looking for an email client that can send delayed email at pre-scheduled times, for reminders, follow up etc.

It seems like email scheduling has disappeared from most mac email clients? except for Thunderbird. It was the only email client I could find that supported pre-scheduled emails.

Is there an email program that can set to send email at specific times and date automatically? Thunderbird seems to work but it needs to be open and active - does not send if you forget to open thunderbird.