Go into Mail as you would to compose an email
Tap the compose button in the upper-right corner
Type your email, and them you realise you need to look in your calendar or do something else
Click "Cancel", then "Save Draft"
When you come back to mail after saving your draft, just do the next couple of steps.
When you come back to your mail, tap and hold the compose button, it will list your draft emails. You can tap the one you want to complete.
Now you can complete your email.
I hope this helps you in speeding up your work 😀
The creator of this guide has not included tools