If an email can't be delivered, it will be shown with a red line around the text and Sending failed in red on the right side. You can hover over the message and see why the message could not be delivered.

The descriptions below are taken from a Mailchimp support article.

Soft vs. Hard Bounces

When an email is rejected by an email server, it's called a bounce. There are different types of bounces that depend on the reason the email bounced.

Hard Bounces

A hard bounce indicates a permanent reason an email cannot be delivered. In most cases, bounced email addresses are cleaned from your audience automatically and immediately. Cleaned addresses will be excluded from all future campaign sends. Here are some common reasons an email may hard bounce.

  • Recipient email address doesn't exist.
  • Domain name doesn't exist.
  • Recipient email server has completely blocked delivery.

Soft Bounces

Soft bounces typically indicate a temporary delivery issue and are handled differently than hard bounces.

 While there are many reasons an email address may soft bounce, these are some common reasons this could happen.

  • Mailbox is full (over quota).
  • Recipient email server is down or offline.
  • Email message is too large.