6 minute read time
Avoiding the Email Naughty List: Why Inbox Filters Get Stricter During the Holidays
When the holidays roll around, inboxes start to look like crowded shopping malls.
Between Black Friday and New Year’s, brands send more email than at any other time of year. Mailbox providers (or MBPs, like Gmail, Outlook, and Yahoo) go into what we like to call “peak vigilance mode.” Their main goal? Protect users from inbox overload and keep spam, scams, and low-quality emails at bay.
Inbox Overload
Between October and December, inbox volume doesn’t just increase; it can triple. That means your subscribers are waking up to hundreds of new messages each day, all fighting for attention. The biggest surge happens between Thanksgiving and Christmas, when brands flood inboxes with Black Friday/Cyber Monday offers, shipping reminders, and deals.
Mailbox providers also know what’s coming. During this stretch, they start tightening filters and limiting how much mail they’ll accept and process per sender per day. It’s their way of protecting users from the inevitable spike in spam and poor-quality campaigns that arrive with the season.
New Senders and Scammers
The holidays are notorious for attracting opportunistic senders and bad actors. Every year, MBPs brace for an influx of new and often untested senders who suddenly start sending emails to capitalize on holiday spending.
Some of these senders are legitimate businesses emailing for the first time. Others may be bad actors, spoofing trusted brands, running phishing campaigns, or blasting massive cold lists.
Because of that, mailbox providers ramp up their monitoring in a few ways:
- Stricter authentication enforcement: During the holidays, MBPS place extra weight on SPF, DKIM, and DMARC alignment. If your sending domain isn’t properly verified, you risk being lumped in with suspicious or spoofed senders.
- Behavioral pattern analysis: MBPs track sudden changes in sending frequency, volume, or audience quality. If a brand that typically sends one campaign a week suddenly sends five, filters may assume the domain has been compromised or is engaging in spam-like behavior.
- Link and domain reputation checks: Filters examine every link and image domain in your message. Redirects, link shorteners, or excessive URLs can trigger extra scrutiny.
- Content similarity detection: A common tactic scammers use is cloning legitimate emails. MBPs use machine learning to detect identical or near-duplicate content across multiple senders and domains.
Even if you’re a legitimate brand, acting like a new one by changing domains, sending from a new IP, or dramatically altering your email program and frequency, can trigger red flags.
Learn how to authenticate your domain with detailed steps in Campaign Monitor’s Deliverability and Authentication Video 101.
The Science of How MBPs Decide What Reaches the Inbox
Spam filtering isn’t made by a random decision. It’s dozens of data points working together, constantly evaluating your email history and engagement.
Mailbox providers use machine learning and natural-language processing to scan every message for risk signals. They’re not just looking for “bad” words; they analyze tone, balance, and intent.
- Red flags: excessive punctuation, all caps, misleading urgency, or emoji overload.
- Technical cues: mismatched URLs, broken links, or image-only emails.
- Safer alternative: “FREE OFFER NOW!!!” becomes “A little holiday gift, just for you.”
Begin testing your emails by sending a quality assurance test to both yourself and a colleague to check all the links. Our Campaign Monitor by Marigold solution also helps you send with complete confidence with its Link Review feature, which automatically detects and notifies you of any broken or missing links in your emails.
The goal is to use language that feels real and matches your brand voice. If your email opens, clicks, and conversions stay strong, that’s proof your content is hitting the mark.
For more information on each of these email tips, visit our Design Best Practices Guide.
Engagement Signals
MBPs watch how recipients interact with your messages across your entire domain, not just one campaign.
- Positive signals: opens, clicks, replies, forwards.
- Negative signals: deletions without reading, spam complaints, marking junk.
And while unwanted, unsubscribes are actually healthy for your email program. They help maintain list hygiene and reduce spam complaints. On average, unsubscribe rates under 2% are within industry norms according to Campaign Monitor’s industry data.
When engagement drops, algorithms assume your content is less relevant, and future messages get filtered more aggressively.
Successful marketers use Campaign Monitor's Insights regularly, constantly tracking campaign reports, and reviewing spam complaints data from MBPs to help paint a vivid picture of performance across different domains.
Even “Good” Emails Can Get Caught in the Crossfire
Even the best-intentioned marketers land in promotions or, worse, spam.
That’s because MBPs prioritize user protection over precision. During peak season, false positives are safer than false negatives. It’s better for them to misclassify one legitimate offer than to let a phishing email slip through.
If your emails look or behave too similarly to mass marketing blasts, they can get swept up in the stricter rules, even if your reputation is solid.
What MBPs Reward This Holiday Season
If filters get tougher, they also get smarter. Here’s what mailbox providers are rewarding right now:
- Steady sending patterns: predictable cadence and consistent frequency.
- Strong authentication: SPF, DKIM, and DMARC properly aligned.
- High engagement: subscribers opening, clicking, and interacting regularly.
- Relevant personalization: timely, valuable content that feels one-to-one.
- Opt-out (and opt-down) options: clear and easy unsubscribe options.
- Balanced design: text and visuals that load smoothly across devices.
In short: MBPs reward trustworthy behavior and consistency is the ultimate gift you can give your deliverability.
Wrapping It Up: Deliverability Is About Trust
When filters tighten during the holidays, they’re simply ensuring that subscribers see what they truly want. If your emails are authenticated, well-timed, and genuinely relevant, you’ll stay on the nice list all season long.
To support you, Campaign Monitor offers Campaign Score, an in-app feature designed to help you evaluate and optimize your email performance. Campaign Score provides a clear rating for your campaign, backed by data, focusing on four key areas:
- Engagement
- Deliverability
- List Health
- Overall Performance
It also includes practical recommendations to help you act on the results, so you can improve campaign by campaign, with full visibility into what’s working and where to adjust.
So before you hit send on that next holiday campaign, take a moment to check your deliverability health. Keep your messages merry, measurable, and exactly where they belong: the inbox.
New to Campaign Monitor? Get started today. Already a customer? Log in now to get a better understanding of your holiday email performance. Also check out our Holiday Delivery Webinar.
