Cold Email Deliverability Audit: Fix Flagged Sequences

You built the sequence. You wrote decent copy. Open rates looked fine for the first two weeks, and then something shifted. Reply rates tanked, bounces crept up, and someone on the team noticed their own test address never got the email. Sound familiar?

Cold email deliverability doesn’t fail loudly. It fails quietly, inbox by inbox, until you’re broadcasting into a void and your domain is silently on every major blocklist. This post is the audit I run when a sequence starts showing those symptoms, using Instantly as the command center and a handful of supporting tools to close the gaps Instantly can’t close on its own.

Why sequences get flagged (and why it’s rarely just the copy)

Most operators assume deliverability problems are a content issue: spam trigger words, too many links, subject lines that sound too salesy. Occasionally that’s true. But in my experience, copy is almost always the last thing to fix. The first three failure modes are infrastructure, full stop.

Google and Microsoft’s filtering logic now scores the sending infrastructure holistically. Domain age, authentication records, inbox reputation, engagement history, and sending volume patterns all feed the decision before a single word of your email gets evaluated. Google’s email sender guidelines require authenticated domains and below-0.10% spam complaint rates for bulk senders. Miss either threshold and you’re filtered at the MX level before any content analysis runs.

The other underrated factor is list quality. Sending to stale, scraped, or unverified lists pushes bounce rates up fast. A 6% hard bounce rate on your sending domain accelerates blacklisting in a way that’s genuinely hard to recover from. The irony is that most teams running this problem have been aggressively growing their contact lists while letting the sending infrastructure quietly degrade behind them.

The audit workflow

The sequence below is the order I actually run these checks. Authentication before list hygiene, list hygiene before sending behavior, infrastructure before copy. Skipping steps or running them in reverse wastes time and masks the real problem.

The infrastructure layer: where to get sending domains and managed inboxes

Instantly handles the campaign orchestration layer well. But it doesn’t provision your sending domains or manage the mailbox infrastructure underneath them. That’s where Maildoso and Mailforge come in.

My strong opinion here: don’t provision Google Workspace domains at scale for cold outreach. Google knows the pattern, and new domains hitting bulk outreach volumes within days of provisioning is a textbook spam infrastructure signal. Go with a managed provider instead.

Maildoso approach:
- Managed domain provisioning
  (aged domains, pre-configured DNS)
- Automatic SPF/DKIM/DMARC setup
- Warm-up included in the mailbox plan
- Connects directly to Instantly
  via SMTP or native integration

Mailforge approach:
- Bulk mailbox provisioning
  at lower per-seat cost
- Good for teams running 50+ inboxes
- DNS setup is semi-guided
- Pairs well with Instantly's
  account rotation feature

Mailforge makes sense if you’re managing a large inbox pool and want to keep per-seat costs down. Maildoso makes sense if you want someone else to handle the infrastructure entirely and just receive a working inbox. Neither replaces the audit steps above. They solve provisioning. They don’t fix bad sending behavior.

Choosing your list verification tool

If you’re running any meaningful outbound volume, email verification needs to be baked into the workflow, not treated as an occasional cleanup task. Here’s how I’d pick between the two main options:

ZeroBounce vs. Bouncer: which verification layer fits your stack

Choose ZeroBounce if

• You're working lists with a high percentage of catch-all addresses and need AI-scored deliverability estimates
• You want an API that plugs directly into your enrichment stack (Clay, Apollo, etc.) for real-time verification
• Your list volumes are large enough to warrant the bulk pricing tiers (100k+ records per month)

From $15/mo Try ZeroBounce →

Choose Bouncer if

• You want a simpler, faster interface for one-off list uploads without committing to a monthly plan
• You're verifying smaller batches (under 50k records) and pay-as-you-go pricing makes more sense
• You need GDPR-compliant verification with EU data residency for European prospect lists

Pay-as-you-go from $8/1k Try Bouncer →

Both tools integrate into a Clay or Apollo enrichment step if you want to run verification earlier in the data pipeline. I’d argue that’s exactly where it belongs: verify before enrichment, not after, so you’re not spending enrichment credits on contacts that will never receive the email. For more on how Reply.io compares to Lemlist on the deliverability and sequencing side, that post covers sequencer-level differences if you’re evaluating platforms beyond Instantly.

What to do when a domain is already flagged

If you’re mid-audit and a domain is already on a blocklist or showing a dramatically low Email Health score in Instantly, here’s the recovery path.

1. Pause all campaigns from that domain immediately.
2. Drop sending volume to zero from flagged inboxes.
3. Let warm-up run in Instantly for two to three weeks at low volume (20 to 30 per day per inbox).
4. Submit delisting requests to Spamhaus, SORBS, and Barracuda, but don’t count on fast resolution.
5. If the domain is under 90 days old and already flagged, retire it. Provisioning a new aged domain through Maildoso is faster than waiting out a recovery cycle on a domain that started life with a strike against it.

The best CRM for outbound teams post covers how to structure contact management so flagged domains don’t bleed into your primary outreach infrastructure. Worth reading if you’re also figuring out where Instantly sits in a broader GTM stack.

The one change that prevents most of this

Here’s the pattern I keep seeing. Teams treat sending infrastructure as a one-time setup. Provision the inboxes, connect them to Instantly, set up the sequence, move on. Then warm-up pauses because of a billing issue. DKIM records reset after a domain transfer. List hygiene gets skipped because the campaign is urgent. The whole thing degrades incrementally until you’re troubleshooting a fully burned domain six months later.

The fix is simple and boring: a weekly infrastructure check built into your RevOps cadence. Pull the Instantly Email Health dashboard. Verify warm-up is running on all active inboxes. Confirm no new domains have dropped authentication records. Run any new contact batches through ZeroBounce or Bouncer before they enter a sequence. That’s fifteen minutes a week. It prevents the multi-hour audit I just walked you through.