Guide Email

Why your emails go to spam, and how to stop it

Every common reason outgoing mail is filtered as spam, ordered from the fixes that matter most to the ones that matter least.

iFastHosting Support Team Updated 26 August 2026 9 min read Intermediate

Mail landing in spam is rarely one single fault — it is a spam filter weighing several signals and deciding the total does not add up to "trust this". Work through the causes below roughly in order, since authentication and sending reputation carry far more weight than anything about how a message is written.

1. Missing or broken authentication

SPF, DKIM and DMARC are the first thing a receiving server checks, and increasingly the first thing several major mailbox providers require before they will even consider a bulk sender trustworthy. Without them, there is nothing proving the mail actually came from your domain rather than someone forging it.

No SPF record at all leaves a receiving server with no list to check the sending server against.
An incomplete SPF record — missing a service that legitimately sends for you — makes that sender's mail look unauthorised even though it is genuine.
No DKIM signature means there is nothing proving the message was not altered in transit.
No DMARC record means there is no policy telling receiving servers what to do about mail that fails the other two, and no reports telling you it is happening.

If you have not set these up, start here — SPF, DKIM and DMARC explained covers what each proves, and the guides to setting up SPF, DKIM and DMARC walk through each one.

Authentication is a prerequisite, not a guarantee

Passing SPF, DKIM and DMARC proves you are who you say you are. It does not, by itself, make a receiving server want your mail — reputation and content still count on top of it, which is why fixing authentication sometimes does not fully solve the problem alone.

2. Sender reputation

Mailbox providers track how recipients treat mail from a given domain and server over time — how often it is marked as spam, deleted unread, or replied to. A domain with a poor history sending from a shared server can be affected by other senders on the same infrastructure, which is one reason a server's reputation matters as well as your own domain's.

A sudden jump in sending volume from a domain that normally sends very little looks like the start of a spam campaign to automated filters, even when it is entirely legitimate.
A high bounce rate — sending to addresses that no longer exist — damages reputation quickly. Clean your mailing list of addresses that consistently bounce.
Recipients marking your mail as spam, even by mistake, is tracked and counted against the sending domain.

3. Being blacklisted

A domain or server IP address can end up on a blacklist that mail providers check against directly, usually as a result of the reputation issues above or, on shared infrastructure, another customer's behaviour. A listed domain is filtered aggressively regardless of how well everything else is configured. See what to do if your domain is blacklisted for how to check and get delisted.

4. Content that trips spam filters

Content matters less than authentication and reputation, but it still counts, particularly for a domain with limited sending history that filters have not yet learned to trust.

Subject lines heavy with urgency or promotional language — excessive capitals, exclamation marks, or phrases strongly associated with spam campaigns.
A large image-to-text ratio, or a message that is almost entirely one large image, which filters cannot read and treat with suspicion.
Broken or suspicious-looking links, including link shorteners that hide the actual destination.
A mismatch between the display name and the actual sending domain, which can look like impersonation even when it is not.

5. Where the message was sent from

Mail sent through a website's default server function — rather than through an authenticated mailbox — is a common cause of poor deliverability, since it is harder to authenticate properly and often lacks the reputation of a real mailbox. See making a contact form deliver reliably if this applies to you.

How to actually diagnose which one applies

Check your authentication first
Confirm SPF, DKIM and DMARC are all published and passing. This is free, quick to check, and the highest-impact fix available.
Look at the message headers on a spam-flagged email
Most mail providers include a spam score or specific reason in the message headers when a message is filtered, viewable through "show original" or similar in most mail clients.
Check whether your domain or server IP is blacklisted
A number of free blacklist-checking tools will test a domain or IP against dozens of lists at once.
Ask the recipient what they saw
If it is one specific person or one specific provider, the message headers from their side often show exactly why it was filtered — something you cannot always see from your own mailbox.

There is no single setting that fixes deliverability permanently

Reputation is monitored continuously by every major mailbox provider, and it can decline again after being fixed if the underlying cause — a bad list, a compromised account sending spam, a missing authentication record — returns. Treat this as ongoing hygiene rather than a one-off task.

Once the causes above are addressed

Deliverability improves gradually rather than instantly, since mailbox providers rebuild trust in a domain over time rather than reversing a filtering decision the moment a fix is made. Keep authentication in place, keep your list clean, and keep an eye on bounce and complaint rates going forward — see cutting down the spam reaching your inbox for the other side of the problem, where your own inbox is receiving too much rather than sending too little successfully.