I have three email accounts that I use two different Mail apps to manage on my iPhone 5 (iOS 10.3.4). I read two accounts with Apple Mail, and one with myMail. This allows me to keep work and personal accounts separate. Unfortunately all three accounts are always available as the sender in Mail even though myMail only sends from the accounts it manages. This means that I sometimes accidentally send mail from the wrong account when using Mail. Is there any way to restrict the accounts that Apple Mail can use, so that Mail can only use the accounts it manages like myMail only uses the account it manages?
As resolved in comments, the account got added in mail app too. It can be removed from Settings → Passwords and Accounts.
For temporary fix, setting a default account under Mail Preferences can help.
Outgoing server's settings can be also disoriented under the Account's Server settings IF it is a work account and allows interface like: