5 Email Deliverability Mistakes Killing Your Cold Outreach

Back in 2016, cold email was simple. "You could plug in a brand new account on a brand new domain, and on day one, send a hundred to 300 emails — absolutely zero problem," recalls Benny Rubin, CEO of Senders and veteran of email deliverability.

But things have changed dramatically. "Mailbox providers have really clamped down on cold email because of the sheer volume," he explains.

After nearly a decade helping companies navigate these changes, Benny has identified key mistakes that consistently hurt cold email campaigns:

Mistake #1: Using open rates to measure campaign success 

Open rates are a go-to metric for measuring email performance, but they're increasingly unreliable.

When security systems scan emails within fractions of a second, bot opens happen, artificially inflating your numbers. Privacy features block tracking pixels, deflating them. Email client updates can swing your rates dramatically overnight.

Many sales teams make critical campaign decisions based on open rate fluctuations that might just be technical noise. Even worse, focusing on open rates can lead to optimizing the wrong things, like flashy subject lines that get opens but don't generate business results.

The fix: Focus on reply rates, especially interested replies, as your primary health metric. These represent actual human engagement and correlate directly with revenue. Use open rates as a secondary indicator, and look for dramatic changes over time rather than absolute numbers.

Mistake #2: Skipping the domain aging process

New domains are like strangers on the internet. They have no reputation, good or bad. Email providers are suspicious of domains that suddenly start sending hundreds of emails with no history.

There's something called a "fresh domain blacklist" that can affect domains for up to 90 days after purchase. This is why advice you see to buy dozens of domains and rotate through them is a hack, not a sustainable strategy.

The fix: Purchase domains well in advance (at least 90 days before use) and warm them up properly. For immediate capacity, use subdomains of existing aged domains. Then make sure you’re setting up your SPF, DKIM, and DMARC authentication. 

Did you know you can get a new domain with mailboxes right in Apollo?

Mistake #3: Hiding or removing unsubscribe links

Some sales teams remove all links, including unsubscribe options, after hearing that "links hurt deliverability." This backfires because unsubscribe links signal responsible behavior to email providers like Google.

The bigger threat is spam complaints. When people can't easily unsubscribe, they're more likely to mark you as spam, which is far more damaging to deliverability than including an unsubscribe link.

The fix: Always include functional unsubscribe links. People who want out will find a way regardless. It's better that they unsubscribe than report you as spam.

Mistake #4: Spiking send volume instead of ramping gradually

Here's a common scenario: The quarter's ending, targets loom large, so the sales team decides to crank up email volume — doubling or tripling daily sends overnight. This is the email equivalent of walking into a bank wearing a ski mask. It doesn't matter if your intentions are innocent; you're triggering all the alarm systems.

Email providers prefer gradual, predictable increases that look like natural business growth rather than aggressive scaling that resembles spam operations. For instance, going from 20 to 30 to 40 emails per day over three weeks signals healthy growth. Jumping from 20 to 160 overnight looks suspicious.

The fix: Avoid sudden spikes. Ramp up linearly by adding the same number of emails daily (5, 10, 20) instead of doubling your volume. This looks more natural and sustainable to email systems.

Check out Apollo’s built-in warmup to warm up brand new mailboxes or domains or handle ramp up for existing mailboxes with sending history.

Mistake #5: Treating catch-all emails like regular addresses

Not all email addresses are created equal.

Catch-all addresses (like info@company.com or hello@company.com) are configured to accept emails sent to any address at that domain—even non-existent ones like randomstring@company.com. These addresses typically belong to larger companies with aggressive security systems.

Sending too many emails to catchalls can damage your sender reputation because these emails are more likely to be flagged or filtered aggressively.

The fix: Segment catchall emails into separate sequences with lower volume limits, ensure they make up less than 2-3% of your daily send volume, or filter them out of your searches altogether.

Cold email still works, when done correctly

The good news? Email isn't dead — far from it. As Benny puts it, "email still works because people are even more phone-addicted and checking their email more often." The channel is thriving; it's just become more discerning about who gets through.

Companies regularly send thousands of emails daily with high deliverability rates. The difference is they understand that email deliverability is a technical discipline requiring proper setup, monitoring, and maintenance.

If you're facing deliverability issues, audit your setup. Are you using aged domains? Are you scaling linearly? Are you following compliance best practices?

Get these fundamentals right, and you'll consistently land in more inboxes.