Sending internal system notifications using the same ESP as transactional emails can negatively impact deliverability. Factors such as shared IP reputation, list hygiene, authentication, engagement, and sender reputation are all affected. High bounce rates, spam complaints, and unauthenticated emails from internal notifications can damage the sender's reputation and subsequently harm the deliverability of transactional emails. Proper email authentication (SPF, DKIM, DMARC), maintaining clean email lists, segmenting mail streams, using dedicated IPs, warming up IPs, and monitoring IP reputation are key to mitigating these risks. Correctly configuring SMTP connections can also improve deliverability.