The increase in hard bounces from smaller domains is a multifaceted issue stemming from strict spam filtering and heightened sensitivity to various factors. Core issues include a compromised sender reputation (affected by spam complaints, low engagement, and potentially listing on blocklists), improper email authentication (SPF, DKIM, DMARC), and poor list hygiene. Sending IPs with ratios of spam to ham across different time windows, or new IPs without a proper warm-up, can also trigger blocks. Content-related problems, such as spammy signatures or the use of certain keywords, are also impactful, as are dramatic changes in sending volumes. Finally, discrepancies between visible and actual “From” addresses and exceeding the smaller sending limits used by some smaller domains are likely to lead to increased hard bounces. The use of SMTP error codes provides key information, and engagement metrics serve as a strong indicator.