iOS 13 - Mail auto-fetch will not disable Since iOS 13 Mail seems to auto-fetch new mail every time the iPhone is plugged into any charging source, either Mac or simple wall-wart.
I've always had my iPhone set to manual Mail fetching for all except my actual Apple iCloud account, which is my only IMAP account &amp; only used for official notifications from Apple.
My main email is done over a POP3 structure, intentionally so I don't have a phone-full of emails. These have always been set to only fetch if I launch the Mail app.
I've always had the main Push* setting on, but then setting each individual account except iCloud to Manual. Switching off this main Push seems to have no effect.
*Settings &gt; Passwords &amp; Accounts &gt; Fetch New Data &gt; Push

This has affected every iOS from 13.0 &amp; continues in 13.6
Edit May 2020 - no fix but a rapid workaround  -
iOS iPhone/iPad Mail - rapid mass delete/delete all
After several weeks of testing - this doesn't work. All it does is prevent the badges, not the actual unwanted mail fetch

What appears to fix this is to turn off Notifications for Mail
Settings &gt; Notifications &gt; Mail &gt; Allow Notifications.
This does mean that you never see Badges if/when any mail at all is received, so you do have to actually launch Mail to see if it worked… which then loads manually, of course.