Missing sent emails to a specific recipient on iPhone 16 Pro and MacBook Pro

I have a new MacBook Pro and a new iPhone 16pro. Since I set them up and transferred data from my older models, the sent mail hasn’t worked properly. I’ve been unable to get it to appear chronologically. Then suddenly, large gaps have appeared in my sent mail, which I keep indefinitely. The missing emails are to one person. Other sent mail seems to be okay. But I write to this friend daily, and a very large number of those emails have vanished. I’ve tried every solution suggested online so far.

[Re-Titled by Moderator]

Original Title: Missing sent mail