Email Deliverability Basics: How Messages Reach the Inbox

Sending an email and having it delivered to an inbox are two different things. A message can be accepted by the receiving server and still be placed in the spam folder, or silently dropped. Deliverability is the practice of making sure that the messages you send actually arrive where a person will see them. It rests on three things: who you are, how you behave, and what you send.

Delivered is not the same as inbox

When your mail server hands a message to the recipient's server, the recipient can respond in several ways. It can reject it with an error, which produces a bounce. It can accept it and place it in the inbox, in a spam or junk folder, or in a category tab. Or, rarely, it can accept and discard it. A report that says "delivered" usually only means the receiving server accepted the message. Inbox placement is decided afterwards, by that provider's own filters.

Pillar one: authentication

Receiving servers want evidence that your message is genuinely from your domain. Three DNS-based standards provide it:

• SPF lists the servers allowed to send mail for your domain.
• DKIM adds a cryptographic signature that proves the message was not altered and was authorised by the domain.
• DMARC tells receivers what to do when SPF and DKIM do not line up with the visible From address, and sends you reports.

Without these, large providers are far more likely to distrust your mail. Major mailbox providers have also introduced stricter requirements for senders who send large volumes, so setting up all three is now the baseline. We cover each in depth in separate articles.

Pillar two: reputation

Mailbox providers keep a running judgement of the IP addresses and domains that send them mail. It is built from signals such as how many recipients mark your mail as spam, how many addresses you send to that do not exist, and whether recipients open, reply to or delete your messages unread. The details differ by provider and are not published, so avoid treating any single number as a rule.

Things that damage reputation

• Sending to purchased or very old lists.
• High bounce rates from invalid addresses.
• Spam complaints because people cannot find how to unsubscribe.
• Sharing a sending IP with other senders who behave badly.
• A compromised account that sends spam without your knowledge.

Things that build it

• Sending only to people who asked to hear from you.
• Removing addresses that bounce.
• Keeping sending volumes steady, and increasing them gradually for a new domain or IP.
• Making unsubscribing easy and honouring it fast.

A brand-new domain has no sending history, so providers have little to judge it by. Start by sending real conversation to people who know you, ask them to reply, and increase volume gradually. Avoid launching a new domain with a large campaign on day one.

Pillar three: content and format

Filters do look at the message itself, though less through single "spam words" than through overall patterns. Some practical points:

• Use a real From name and a reply address that works.
• Match the links in your message to your own domain where you can. Many different link-shortening domains can look suspicious.
• Avoid an image-only message with no text.
• Include a plain-text version alongside HTML.
• Keep the subject line honest. Misleading subjects trigger complaints.

Infrastructure details that matter

Item	Why it matters
Reverse DNS (PTR)	The sending IP should resolve to a hostname that matches forward DNS.
TLS between servers	Modern providers expect encrypted delivery where possible.
Consistent sending domain	Switching domains constantly resets trust.
Blocklist status	Being listed on a widely used blocklist can cause rejections.

How to monitor deliverability

1. Read your DMARC aggregate reports. They show which servers send as your domain and whether they pass.
2. Check bounce messages for the reason codes. A permanent failure for a bad address differs from a temporary deferral.
3. Send test messages to accounts at several large providers and check the headers for authentication results.
4. Watch for a sudden drop in replies or in website form submissions, which can be a sign that mail is going to spam.

Separate your mail streams

Day-to-day staff mail, newsletters and automated website messages have very different risk profiles. A newsletter that gets complaints should not harm the reputation of the domain your accountant uses to send invoices. Many businesses use a subdomain for bulk or automated mail so that problems stay contained. Our article on transactional email explains this for website messages.

Deliverability with Zebra Hosting

A reliable mailbox host is the starting point for good deliverability, because your records and server need to be configured correctly. Zebra Hosting's email plans run on Plesk, and support in 17 languages can help you check your DNS records when something looks wrong. Explore the business email plans, and see the security page for how we think about protecting your domain.