How email actually gets delivered

Between pressing send and someone reading, several systems make independent decisions. Most email problems are a misunderstanding of which one said no.

The path

1. Your email service provider (ESP) accepts the message.
2. It hands off to the receiving mail server for that domain.
3. The receiver checks authentication — is this sender who they claim.
4. The receiver checks reputation — do we trust mail from this sender.
5. The receiver decides: inbox, spam folder, or reject.

Every step can fail differently, and only some failures are visible to you.

Delivered is not delivered-to-inbox

This is the single most important distinction in the channel. Your ESP reports "delivered" when the receiving server accepted the message. It has no idea whether it went to the inbox or straight to spam.

A campaign can show 99% delivered and 0% inbox placement. The dashboard will look fine. So: delivery rate is a hygiene metric, not a success metric. Inbox placement is what matters, and measuring it needs seed testing or provider tools.

Reputation is per-sender, per-receiver

Gmail forms its own opinion of you; Outlook forms a different one. They do not share. This is why "we have a deliverability problem" is almost never true as stated — you have a Gmail problem, or an Outlook problem, and the fix differs.

Always segment your metrics by receiving domain. This is the highest-value habit in email and most senders never do it.

What reputation is built from

• Engagement — opens, clicks, replies, moving mail out of spam. Positive.
• Complaints — "mark as spam". Very negative, and the threshold is low.
• Unknown users — sending to addresses that do not exist signals a poorly-maintained list.
• Spam traps — addresses that exist only to catch senders who did not get permission.
• Consistency — sudden volume spikes look like a compromised account.

Reputation attaches to both your sending domain and your sending IP. On shared IPs you inherit your neighbours' behaviour, which is a real argument for a dedicated IP at volume — and a real argument against it below volume, since a dedicated IP with low traffic never builds reputation at all.

Bounces

Hard bounce — address does not exist. Remove immediately; continuing to send is a strong negative signal. Soft bounce — temporary, retry a few times then stop.

What trips people up

• Reporting delivery rate as if it meant inbox placement.
• Averaging across providers and hiding a Gmail-specific collapse.
• Emailing an old list after a long gap, which looks exactly like a compromised sender.

You have got this when

You can explain why a 99% delivery rate is compatible with nobody seeing your email.