Doubt is a normal, natural part of life, even for people of faith. Doubt is healthy and can act as a nice brake pedal to our quest to do things on a whim and go faster and faster than ever.
That's why I like this point made about doubt and emails. When in doubt about sending the email, don't do it. You'll surely regret it.
Doubt is not the enemy of faith; it is the preventer of regret.