Email Provider is a big one - particularly following best practices like DKIM. Use a large managed service like Postmark or Mailgun. Use AWS SES/roll your own

Email Provider is a big one - particularly following best practices like DKIM. Use a large managed service like Postmark or Mailgun. Use AWS SES/roll your own at your own peril. Worst case, the user doesn't get the email, and uses OAuth (majority of my target audience - agencies - use GSuite).