Blocks happen when your email is rejected because of an issue with the message itself rather than an issue with the recipient's address.
There are several causes for blocked emails. For example, your mail server IP address may be blocked by an ISP, or the receiving server may flag the message content using a filter. Twilio SendGrid will not suppress future messages to blocked addresses by default.
For more information, please see our Blocks documentation.
You can also see your Blocks in the Suppressions settings menu of the Twilio SendGrid App.