Check your Spam Inbox
Sometimes a preview will wind up in your spam folder. Make sure to flag it as not spam!
Check if the email is on a bounce/unsubscribe list
For whatever reason, your email may have been accidentally added to your ESP’s block/unsubscribe list. An easy way to check for this is by sending a preview to your email address with a label
Check your sending domain
Some ESPs (like SparkPost and Postmark) require you to have a verified sending domain. Ensure that your sending domain matches the From address in your Sendwithus Delivery Settings
Send a real email
Use a tool like Insomnia to make an API request. The resulting email log may tell you why the email is failing.