If you've upgraded to iOS 13 then you may have discovered that the Mail app is behaving a little oddly, which is a bit of a nuisance if you rely on this app for work.
The most annoying bug seems to be the "no sender" bug, where messages in the inbox shows "No Sender" as opposed to the sender's name for some users. This can turn an already chaotic mailbox into an unmanageable mess.
A fix for this is to kill the Mail app from the multitasking tray and then reboot the iPhone. This should fix the issue, at least temporarily, and should buy you time until the next update lands.
Another interesting bug is that the notifications badge on the Mail icon is erratic and unreliable. There's no workaround for this, so it's a case of grinning and bearing it until Apple gets around to fixing it.
Another reported issue is the Mail app not retrieving email. Some users report that deleting the mail accounts and recreating them fixes these, but I've also seen reports of some people saying this hasn't helped, so it might be worth waiting for an update.
Hopefully, iOS 13.2, which is scheduled to land Sept. 24, will fix these issues.