Email Deliverability
Why Your Client's Email Campaign Failed (And It Wasn't the Copy)
When a client's email campaign underperforms, the instinct is to blame the subject line or the list. Most of the time the real problem is infrastructure. Here's how to diagnose it.
The campaign went out. The numbers came back bad. Open rate at 8% when it's usually 28%. Bounce rate spiked. The client wants answers.
The first instinct theirs and sometimes yours is to look at the creative. Was the subject line weak? Did the list go cold? Did the send time matter?
Those questions are worth asking. But before you get to them, check the infrastructure. The majority of sudden deliverability drops are caused by something upstream of the message a DNS record that broke, a domain that appeared on a blacklist, an authentication failure that started routing email to spam before anyone noticed.
Here's how to diagnose it.
Step 1: Check the bounce data first
Hard bounces and soft bounces tell different stories.
A spike in hard bounces (permanent failures 5xx SMTP codes) usually means one of three things: the list has gone significantly stale and you're hitting invalid addresses, the sending domain or IP is being actively rejected by receiving mail servers, or there's a DNS configuration problem causing mail server lookup failures.
A spike in soft bounces (temporary failures 4xx SMTP codes) often means the sending IP or domain is being throttled or temporarily blocked. Receiving mail servers use 4xx codes when they're suspicious of a sender but not ready to permanently reject them.
Pull the full bounce log from your ESP. Look at the SMTP response codes and error messages. They'll tell you what the receiving server actually said.
Step 2: Run a blacklist check immediately
The single most common cause of sudden campaign failures is a blacklist listing.
Email blacklists are databases of domains and IP addresses known to send spam. Receiving mail servers check inbound email against these databases and reject or spam-folder email from listed senders. Getting listed even on a minor blacklist can significantly impact deliverability. Getting listed on a major one like Spamhaus can effectively stop email delivery to large portions of the internet.
Check both the sending domain and the sending IP. Your ESP's sending IP is often shared with other senders, which means another sender on the same IP getting listed affects your client too.
The major blacklists to check: Spamhaus (SBL, XBL, PBL), Barracuda, SORBS, SpamCop, URIBL, SURBL.
If you find a listing, don't panic. Most blacklists have a delisting process. The key is finding it fast every hour on a blacklist is another hour of degraded deliverability.
Step 3: Validate the authentication stack
Authentication failures are quiet. They don't generate hard bounces or error messages your client will see. Email just silently routes to spam.
Check three things:
SPF Does the record include the ESP that sent this campaign? If the client changed ESPs recently, added a new email tool, or if the DNS was modified for any reason, the SPF record may no longer cover the sending source. An SPF failure combined with a DMARC policy of p=quarantine or p=reject means the email goes to spam or gets rejected entirely.
DKIM Is the DKIM signature present and valid in the email headers? Pull a raw email from the campaign and look at the DKIM-Signature header. Check the selector against the DNS record. A DKIM failure on its own may not kill deliverability, but combined with an SPF failure it almost certainly will.
DMARC What's the current DMARC policy? If it changed since the last campaign even if SPF and DKIM are passing a misconfigured DMARC alignment could be causing failures. Check p=, pct=, and adkim=/aspf= alignment settings.
The authentication check your ESP's campaign report shows you is not always accurate. Pull the raw email headers and check the authentication results directly.
Step 4: Check for recent DNS changes
DNS changes are invisible until they break something.
Ask the client: has anything changed in their DNS recently? New website, new hosting, domain transfer, IT migration, new email tool? Any of these can modify DNS records that affect email delivery.
Also check for changes you didn't authorize. Nameserver changes are an early indicator of domain compromise. If the nameservers changed without anyone knowing why, treat it as a security incident.
Check the domain's TTL and when records were last modified. Some DNS providers show change history. If they don't, compare the current records against a recent screenshot or backup if you have one.
Step 5: Look at engagement history before the campaign
Gmail and other major mailbox providers use engagement signals to determine inbox placement. A domain or IP with a history of low engagement emails that get ignored, deleted without being opened, or marked as spam accumulates a negative reputation that affects future campaigns.
Before blaming the most recent campaign, look at the trend. Has open rate been declining gradually over the past several sends? That suggests a reputation problem building over time, not a sudden infrastructure failure.
If engagement has been declining gradually, the infrastructure checks above may all pass the problem is sending reputation, not configuration. The fix involves list hygiene, re-engagement campaigns, or reducing send volume while reputation recovers.
If the drop was sudden fine last week, broken this week it's almost always an infrastructure or configuration change.
Step 6: Check the receiving domain mix
Not all deliverability problems affect all recipients equally.
If open rate dropped at Gmail but not at Outlook, the problem is likely Gmail-specific reputation signals, Google bulk sender compliance, or a Gmail-specific blacklist.
If it dropped across all mailbox providers simultaneously, the problem is more likely at the sending end blacklist, authentication failure, or a DNS issue.
Your ESP's campaign analytics should break down delivery and engagement by receiving domain. If it doesn't, pull a sample of the bounce logs and look at which domains are bouncing.
The infrastructure checklist for a failed campaign
When a campaign underperforms, run this in order before touching the creative:
- Pull bounce log hard vs soft bounces, SMTP error codes
- Blacklist check sending domain and sending IP
- SPF validation does it cover the sending ESP?
- DKIM validation is the signature present and valid in raw headers?
- DMARC check current policy,
pct, alignment settings - DNS change check any recent modifications to the domain's records?
- Nameserver check are they what you expect?
- Engagement trend sudden drop or gradual decline?
- Receiving domain breakdown which mailbox providers are affected?
If you run through this list and everything checks out, then look at the creative, the list, and the send cadence. But in our experience, the infrastructure check finds the problem before you get to step three more often than not.
How to avoid this next time
The reason these problems are hard to diagnose after the fact is that most agencies aren't monitoring before the campaign goes out. By the time the numbers come back bad, the window for easy fixes has closed. The campaign has already run. The deliverability impact has already happened.
The agencies that avoid these situations are running continuous monitoring checking authentication, blacklist status, DNS configuration, and SSL every hour across every client domain. When something breaks, they know within minutes, not after the next campaign send.
It's the difference between calling the client to say "we caught a blacklist listing this morning and got it removed before your campaign went out" and calling them to say "we're investigating why the campaign underperformed."
One of those calls builds client trust. The other costs it.