emails on all devices

Whether or not emails are deleted on all devices depends on the type of email account that you have. IMAP email accounts will synchronize across all devices. POP email accounts will not. Apple has no control over email accounts other than @icloud.com emails, which are IMAP and do sync.

JRuth2021 wrote:

Thank you, Idris, for your clear answer. When you consider all the other amazing accomplishments the people at Apple are constantly mastering, it seems that there might be some way around this problem.

Again, this is not an Apple issue. The ability to have emails sync exists. You just need to chose the appropriate service. If your current email account is not an IMAP account, contact your email provider and ask if it's possible to change.