Deliverability

Email deliverability is the ability of an email message to reach the recipient's inbox instead of being rejected by the mail server or diverted to the spam folder. It is the single most important operational metric in email marketing and transactional mail alike — an unopened password reset or order confirmation that never made it past the filter is a direct business loss, not just a statistic.

How deliverability works

Receiving providers such as Gmail or Microsoft 365 score every incoming message against dozens of signals before deciding where to place it. The decisive factors fall into four groups:

• Authentication — valid SPF, DKIM and DMARC records published in DNS prove the message genuinely originates from the domain it claims.
• Sender reputation — the track record of your sending IP address and domain: spam complaints, bounce ratios, blacklist (RBL) listings and sending volume consistency.
• List hygiene — repeatedly mailing dead addresses (hard bounces) or hitting spam traps signals a careless sender and erodes reputation quickly.
• Content and engagement — subject lines, text-to-image ratio, link quality, plus behavioural data such as opens, replies and "mark as spam" clicks.

Deliverability in practice

A practical deliverability programme starts before the first campaign: publish authentication records, set a correct reverse DNS (PTR) entry for the sending IP, and warm the IP up gradually by increasing volume over several weeks. If you run your own mail stack on a VPS, verifying the IP against major blocklists should be part of routine server maintenance.

Ongoing work then focuses on the mailing list itself: remove hard bounces immediately, suppress chronically inactive subscribers, and prefer double opt-in signup so nobody receives mail they never requested. Monitor inbox placement rate, bounce rate, complaint rate and open rate over time — a sudden drop in any of them is an early warning that reputation repair is needed. Tools such as Google Postmaster Tools and aggregated DMARC reports show exactly how the large mailbox providers perceive your domain.