There are several reasons why recipient servers block emails:
IP Address Reputation: If your sending IP is associated with spam, it is likely to be blocked. Content Issues: Emails containing certain keywords or phrases may be flagged as spam. Authentication Problems: Lack of proper SPF, DKIM, and DMARC configurations can result in blocks. High Bounce Rate: A history of high bounce rates can lower your sender reputation. Blacklistings: Being listed on common spam blacklists can cause blocks.