Okay, back again. Not to confuse things here but I tried a slightly different bent on the topic.
I sent a delayed email to my wife on my iPhone (iOS16) using my gmail account set to send at 0800.
View attachment 37217
You can see the 1 item in the now visible Sent Later Mailbox.
Here it is;
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Now, I went to my MBP and checked Mail. No Send Later Mailbox and no new mail in Drafts. I think this is a bit of an issue. I expected that creating a Delayed Mail item in iOS would have resulted in a Send later mailbox appearing in All Mail in macOS Mail.
At 0800 I check my wife's Mail app and my email has been received.
Check my iPhone and the Send Later Mailbox is gone
but my email is not in sent mail.
Check my MBP and here's the big reveal. In macOS Mail my mailboxes are combined, so eg All Inboxes, All Drafts, All Sent etc.
There is my email in All Sent, BUT
not from gmail but iCloud!
I now go back to my iPhone, where the Mailboxes are
not combined and look in iCloud Sent Mail and sure enough there is my email, sent at 0800 as intended.
View attachment 37219
So, it's apparent that Delayed Mail is sent via iCloud (see above) no matter which email provider account is used to write the original.
I tried again with my Microsoft account and the same result.
Obviously this means that if I look in gmail > sent mail it does not appear but as I only use the combined (All Mailboxes) view in macOS it does, not so on my iPhone of course.
If I do a search in macOS Mail for Sent to my wife in
All Mail its there but
not in gmail.
It's possible that some email accounts may not work smoothly with this system but it works perfectly for me.