It seems to me that this is not in the device you are using, but in the mail itself. Now all our information is not safe. Therefore, it is important that all data remain in place. I use https://personal-mail-archiver.com/ for this. Fortunately, I had no more questions like this.
Apple is well-known for it's quality products. iPhones biggest power is it's iOS which becomes more powerful because of it's hardware which is build only to stabilise and boost iOS performance. It's hardware and software compatibility makes it run much smoother than any other mobile OS available currently.