Quick Compose
Quick Compose opens a compose window from any app. Magimail does not need to be on screen, and its window does not come up behind the compose window, even when it was closed.
Shortcut
Section titled “Shortcut”Settings → General → Quick compose hotkey sets it:
Off- ⌥⌘C, the default
- ⌃⌘C
- ⇧⌘C
- ⌃⌥⌘C
Every entry ends in C, and only the modifiers differ, so a chord another app already holds has three alternatives. See the Settings Reference.
Account Selection
Section titled “Account Selection”One account, and the message is written from it. With more than one, Magimail asks which to use before the compose window opens. The answer does not change the account the main window is showing; Multiple Accounts covers the list.
Unavailable Shortcut
Section titled “Unavailable Shortcut”macOS gives a shortcut to the first app that asks for it, so another app may already hold this one. Magimail leaves the setting where you put it rather than switching itself off. Choose one of the other four.
See Keyboard Shortcuts for the rest.