Mailbox providers such as Gmail and Outlook decide where each message goes using several signals. They check authentication through SPF, DKIM, and DMARC, look at the reputation of the sending domain and IP address, and watch how recipients react: opening, replying, deleting, or marking as spam. Messages sent to dead addresses, sudden jumps in volume, and misleading content all count against the sender.
Poor deliverability is easy to miss because often nothing bounces; the quote or invoice simply sits unseen in a spam folder. For a small business that means lost sales and late payments with no error message to investigate. It affects ordinary one-to-one mail as well as newsletters, because both draw on the reputation of the same domain.
Authentication is the foundation but not the whole answer. Send only to people who expect your mail, remove addresses that bounce, make unsubscribing easy, and consider a separate subdomain for marketing mail so it is less likely to harm everyday email. New domains need a gradual increase in volume. Be wary of anyone promising guaranteed inbox placement. We audit every service that sends mail for a domain and fix the DNS records behind it.