Re: "Mail doesn't send emails scheduled for later"
Seems your recipients never get their emails and they remain in the outbox
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Re: "Emails scheduled to be sent later remain in the outbox, sometimes after the recipient receives them".
Seems some of your recipients do get their emails and yet the emails also remain in the outbox
So I guess you have no way of knowing whether or not an email was actually "sent later",
except in some cases where you see the recipient replies, and you then know for sure the email was sent.
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Because use of the "send later" feature is so uncertain for you:
You may want to send feedback to reach those who work on the software behind-the-scenes at Apple.
All feedback is carefully read and logged, and if others are also experiencing a similar issue we may see an update that addresses the issue. The more feedback on which Mac models and Mac OS versions are affected
the more likely we will see it "fixed" in an update. You can include a link to this discussion thread if you wish.
Web-form for feedback: Feedback - Mail - Apple