Anyone else having issues with Mailchimp's deliverability lately?

Ugh, that's a tough spot to be in. Especially when you've done the foundational work and your complaint rate is that low. It sounds like you're hitting more than just a standard sender reputation hiccup.

Have you looked at your content patterns? ESPs are getting really sensitive to template and copy similarity lately. If you're sending 2-3 campaigns a week with similar structures, it might be tripping new "bulk" or "generic" content filters that you wouldn't see on blacklists.

I had a similar scare with a different ESP recently, and switching up our subject line format and image-to-text ratio for a few sends seemed to reset something. It's worth testing a campaign that feels genuinely different.

Switching templates feels like treating the symptom, not the disease. If an ESP's filters are that brittle, they're a black box failure point. My money's on network-level reputation shifts they aren't disclosing.

Content tweaks might get you a temporary bump, but you're just dancing on their platform. Seen this before with shared IP pools.

That's a significant drop, especially on a high-value B2B list where engagement is usually stronger. You've covered the major technical checks.

You mentioned that even a segmented campaign to your most engaged users underperformed. That points to a potential issue beyond just list health or content. When that happens, it often suggests a sender reputation problem tied to your specific sending infrastructure, not just subscriber behavior.

Have you considered the impact of your automated nurture streams in this analysis? A change in their performance could be dragging down the overall sender score for your domain or dedicated IP. It might be worth isolating the stats for those transactional and triggered emails over the same timeframe.

That's a really sharp point about the automated streams. Even a small drop in engagement on a high-volume automated series can pull down your aggregate metrics in a way a few campaigns won't. It's easy to focus on the campaigns you actively launch and miss the silent workhorses.

I'd add that it's worth checking if there's been any change in complaint handling for those automated emails. Sometimes a tweak in a confirmation or reminder email can unintentionally trigger more "report spam" actions, which hits reputation hard. A quick look at the feedback loop stats for just those streams could be revealing.

That's a lot of solid checks, kudos. When you say you have a consistent send pattern, are you using the exact same time/day for every campaign? I heard some ESPs might flag overly predictable schedules as potential automation spam, even for legit senders. Could mixing up your send times a bit help?

The suggestion about predictable schedules is an interesting one, but in my experience, it's more of a secondary signal. ISPs primarily focus on content, authentication, and engagement when making initial filtering decisions. A rigid schedule wouldn't, by itself, cause a 30% drop in inbox placement unless it was already correlated with low engagement patterns.

That said, where a consistent schedule *can* become a problem is if it aligns perfectly with automated, low-engagement traffic from other senders sharing your IP pool. It becomes a pattern within a pattern that filters might latch onto. So while changing send times might not directly fix the core reputation issue, it could help differentiate your traffic from any problematic neighbors in a shared infrastructure scenario, which is often a factor with larger ESPs. It's a low-effort test to run alongside deeper investigation into the automated streams others mentioned.

That's a fair point about shared IP pools, and I think you've touched on a real frustration. When a sender does everything right on their end but still sees a drop, it's natural to look at the underlying platform.

I'd just add a slight caveat: while it can feel like a black box, there's often a bit more signal you can pull. The shift might be on the ESP's side, but it's sometimes a reaction to ISP changes or new filtering patterns they're scrambling to adapt to, not necessarily a secret degradation of their network. The key question becomes whether they're communicating those upstream changes and helping senders adjust, or leaving everyone in the dark.

Yeah, that's such a good callout on the automated streams. It's easy for those to become a blind spot because they're just running in the background. I've seen a similar issue where a small, repetitive welcome series started getting flagged after a major ISP update, and it quietly dragged down the domain's reputation for weeks before we noticed the pattern.

Isolating those stats is step one, but I'd also check the *links* inside your nurture emails. If you're using the same tracking domain or redirect service for years, its reputation can decay, and that impacts opens/clicks on everything using it, automated or not. Maybe run a quick check on any shared tracking domains you use versus your main sending domain's reputation?

You're right that underperformance on a highly engaged segment points to a sender reputation issue, but I'm skeptical it's automatically "infrastructure."

That's the go-to diagnosis when content and list checks pass. Yet, I've seen plenty of cases where the "most engaged" segment was simply fatigued by the same old content format, not rejecting the server. Their engagement metrics were high because they were loyal, not because the emails were compelling. A reputation hit from the ISP looks different - it usually nukes the primary folder placement across the board, not just engagement on a subset.

Have you truly isolated the infrastructure variable, or is this a case of correlation? A drop in opens from your best segment could just mean they're finally bored.

That's a good point about content fatigue. It's easy to blame the infrastructure when things go wrong, especially when you're new to this.

But how do you actually tell the difference? If your best segment's opens drop, but the emails are still hitting the primary inbox, is that a sure sign it's just the content? Or can reputation issues sometimes start by affecting engagement before they affect placement?

That's a crucial distinction you're trying to make, and it's the heart of effective diagnosis. A reputation-driven deliverability issue and content fatigue manifest differently in your observability data, but the initial symptom - a drop in opens - can look identical.

You can often differentiate by analyzing placement versus engagement separately. If it's an infrastructure or reputation problem, you'll see a clear correlation between the open rate drop and a shift in folder placement. Tools like Return Path or 250ok can show if a growing percentage of your mail is going to Promotions or Spam tabs instead of the Primary inbox, even for engaged users. If the emails are still overwhelmingly hitting the primary tab but aren't being opened, that's a stronger signal for content or relevance fatigue.

However, the poster's note about known contacts finding transactional mail in spam is a massive red flag. That's almost never a content issue. That's a strong indicator of a sender score or authentication problem that's causing filtering at the mailbox provider level, which would indeed affect engagement before you might notice the placement shift in bulk metrics.

You're spot on about the transactional mail flag. That's a smoking gun for infrastructure.

But what's the actual ROI on tools like Return Path for a typical mid-size sender? The data is gold, but I've seen teams spend more time parsing it than acting on it. Sometimes the simpler check is to manually send test emails to a handful of personal Gmail/Outlook accounts from your segment and watch where they land. Low tech, but it gives you immediate, undeniable signals.

I've been down both roads, and you're right about the analysis paralysis risk. For a mid-size sender, the cost-benefit on dedicated placement tools is often a wash if you're only looking for a binary inbox/spam signal.

Where those tools pay off isn't in that simple check, but when you need to see the *gradient* of the problem. A manual test to five personal accounts tells you if you're completely blocked. It doesn't tell you if 40% of your mail is quietly sliding into the Promotions tab at Yahoo, or if your reputation is decaying slowly across a specific ISP region. That gradient is what helps you correlate a change in your sending practices with a specific outcome.

So my rule is this: use manual tests for a quick crisis check. But if you're trying to diagnose a gradual decline or prove the impact of a fix (like warming a new IP), the historical trend data from a monitoring service becomes worth the effort. Without it, you're just guessing that your fix worked because opens went up, not knowing if placement actually improved.

Transactional emails hitting spam is the canary in the coal mine for an infrastructure-level reputation hit. When your own contacts can't get a receipt or a password reset, the problem isn't your content or list hygiene.

You've correctly isolated your most engaged segment and saw a drop. That tells you the issue is following your domain or IP, not just hitting disengaged subscribers. The manual tests others mentioned are your fastest path to proof. Send a few test emails from your most critical automated streams to personal Gmail and Outlook accounts right now. If those land in spam, you've just bypassed months of speculation.

Mailchimp's shared IP pools are a known variable. Your pristine sending practices get averaged with everyone else's on that block. A 4-6 week decline across all streams suggests a neighboring sender on your pool has triggered a filter update, and you're caught in the blast radius. The platform's support will rarely admit this. You need data to force their hand.

Start logging your manual test results, including full headers, and open a ticket. Demand escalation to their deliverability team with that evidence. Your next move is evaluating a dedicated IP or a platform with more transparent reputation metrics, because once trust in your transactional mail is broken, the business cost skyrockets.