If you use Mail.app for iOS with several different mail accounts, here are two settings that are very useful.
This is a less commonly known option. Go into the Mail settings and look at the "Mark Addresses" option. Setting a domain here will cause the iOS Mail app to highlight any address not from that domain in red when you are drafting messages.
This is just an example. I set this to include my work domain so it's immediately obvious if I'm sending a message to someone outside of our organization. More importantly, it's immediately obvious if I'm about to send a message from my personal email account.
Swipe to Archive
Here's another one that I use occasionally. Change the swipe option for Mail.app to use "Archive" and it will send the message to the top level Archive folder of the appropriate account.