How to avoid spam filters and improve sender reputation

In this guide, we explain how spam filters work, what affects your sender reputation, and which best practices help improve deliverability.

• Why emails land in spam
• What signals do spam filters use
• What factors decrease spam rate
• Key things to follow - checklist best practices
• What’s the impact of spam filters

What is a spam filter: Why emails land in spam

Deliverability is a foundation of any cold email campaign.

Even good offers can get lost when spam filters block your outreach before it reaches the leads inbox. Providers use spam filters to check each email to decide whether your email goes to the inbox or spam.

What signals do spam filters use

Gmail and Outlook are no longer using simple spam-word filters. Spam filters now use more advanced checks and machine learning to detect unwanted email.

Besides your domain reputation and authentication setup, they also check your sending patterns and limits, past engagement, and the content of your emails.

All these checks create your email’s spam score, which determines whether your email goes to spam:

• AI-based filtering

Gmail and Outlook use AI to check your email’s tone, context, and structure. If your message looks similar to emails that were previously flagged as spam, it can be marked as spam too.

• Behavioral analysis

Providers also track your engagement history and how recipients interact with your emails. Opens, replies, deleting without reading, and spam complaints all affect your sender reputation. If your past campaigns had low engagement, it can hurt the deliverability of future emails.

• Sending patterns (limits)

Providers look for unusual or suspicious sending behavior. For example, if you suddenly send 100 emails after a long period of inactivity, that can look like spammer behavior.

What factors help avoid spam filters

Here’s what matters the most for sender reputation: technical setup, domain health, warm-up, sending limits, email content, replies and bounce rate.

Base requirements (don’t skip)

These are the basic requirements for every sender. If you don’t follow them, your emails have a much higher chance of landing in spam.

Technical setup

Domains & mailboxes

Free mailboxes on @gmail won’t work for outreach. Also, don’t send cold emails from your primary business domain.

Set up separate outreach domains and give them some time to age. Sending from a new domain right away can make it look spammy.

• Buy domains at least 30 days before use (even before warm-up)
• For Outlook senders, new domains have very low trust. Use domains with 1+ year age
• For Gmail senders, relatively new domains can be used with complete DNS setup

Domain (DNS) health

Email authentication is required to stay compliant as a sender. If your DNS records are incomplete, your emails are much more likely to land in spam.

Set up SPF, DKIM, and DMARC records for all your sending domains. Use our built-in Domain Health checker to confirm whether this is already configured.

Daily sending limits

A high sending limit per mailbox is one of the most common signals that can trigger spam filters.

Set up sending limits in the email account settings.

Start with 5 emails per day to verified contacts, then increase gradually by 2 emails per day until you reach 30 emails per account per day.

Connect more mailboxes and use mailbox rotation to increase your total volume safely.

Our recommended limits per mailbox are:

• 5 new emails/day: use as your starting limit for new domains. Also an optimal limit for Outlook / Microsoft senders
• 15 emails/day: optimal start limit for existing Gmail senders.
• 30 new emails/day: for warmed-up accounts. Use this limit only if your reply rate is already 5% or higher.
• 50 emails/day: Risky limit even for accounts with good reputation. Don’t use it regularly, only as a temporary increase.
• 100 emails/day: This is a spammers limit. We don’t recommend using it.

Email Warm-up

Warming up your email accounts is one of the best things you can do to avoid spam filters.

Add your email accounts to Email Warm-up.

It increases your sending volume gradually and builds reputation through positive signals like automatic opens and replies.

Warm up every sender account before using it for a campaign. Both new and existing email accounts should be warmed up.

Minimum warm-up period:

A warm-up needs to run long enough to have a real impact.

• 1-2 weeks: After a pause in outreach or to fix existing issues with deliverability
• 4 weeks: new domains and new mailboxes 

Recommended warm-up settings:

• Turn on the Premium sender pool to warm up using business email addresses
• Enable provider specific warm-up to improve deliverability by provider (Gmail/Outlook)

Warm-up metrics:

Before you launch campaigns, check the warm-up progress on the warm-up dashboard

• If inbox score is lower than 80% → extend warm-up until the score improves
• Deliverability is consistent (90% and higher) by provider (Gmail/Outlook)

Email content

Spam filters check every part of your email, including the subject line, text, links, attachments, HTML code, and number of images.

Format your emails like a casual conversation (unless a formal tone is required). Avoid unnecessary styling or visual elements.

Formatting

• Enable plain-text mode for the first email in your sequence. HTML can negatively affect deliverability

Images/links/attachments

• No images, or links in the first email
• No attachments in the first email
• In the follow ups, you can use one of the elements per email if needed: 1 image/video or 1 external link/1 attachment
• When adding links, make sure they use https:// (not http://). Don’t hide links behind text like “Click here.”

Subject line & Email body

• Subject line = up 5-6 words. Prospect name or company name in subject line increases opens
• The initial email up to 50-100 words
• 20-50 words for the follow-ups
• One clear CTA
• No spam triggers (like "free" or "guaranteed")

Unsubscribe link

Not having an opt-out option can increase spam complaints. Make it easier for prospects to opt out than to click Mark as spam.

• Add an unsubscribe link in email account settings

Reply rate

Reply rate is a strong positive signal for sender reputation because it shows your emails are targeted, not spam.

Aim for a reply rate of 5–10%. To improve your replies, work on your ICP and define your strongest selling points.

Email only decision-makers or contacts whose role matches your offer. Sending the same email to multiple prospects at the same company can trigger spam filters.

Here’s how Snov.io can help with this part of the workflow:

• ICP Generator to create customer profiles for your product
• Database Search to find prospects who match your ICP description
• Email Writer to write email templates based on ICP and selling points

Bounce rate

Bounce rate is one of the clearest signals of list health. Even a small number of bounces can hurt your sender reputation.

Aim for a bounce rate of 5% or lower. To reduce bounces:

• Verify your email list before every campaign
• Make sure your list is healthy, with mostly green verified emails
• Remove any bounced contacts from your campaign right away
• Add them to your Do-not-send list

Bounce rate 1–2%: Acceptable, but list health needs attention. Add yellow (unverifiable) emails only in small batches.

Bounce rate 3–5%: Risky for sender reputation. Don’t add more yellow emails until the rate goes down.

Bounce rate 15% or more: Remove all yellow emails from the list.

Additional tips (good to have)

Following these recommendations will help improve your chances of getting into the inbox.

Provider matching

Emails sent within the same provider inboxes will have better deliverability.

Diversify your email accounts between providers. If you can only use one provider, we recommend Google Workspace.

• Mix your mailboxes: optimal combo is 70% Google mailboxes / 30% Microsoft mailboxes
• Segment prospects by provider. Snov.io automatically detects the provider and shows it in your prospect list.

Campaigns + warm-up (deliverability protection)

Long breaks in sending can hurt your reputation. Warm-up helps maintain your reputation at a high level.

During active campaigns, your daily sending limit should be split between campaign emails and warm-up emails.

Use warm-up as a background process to protect deliverability, not just a one-time step. Keep it active during and between campaigns.

• When campaigns are active, split your daily limit between campaign and warm-up
• When campaigns are paused, use your full daily limit for warm-up
• On days when you send fewer real emails than usual, add more warm-up emails

Leave part of the account’s limit for warm-up. Warm-up and campaign limits are set separately in the settings, but both count toward the total daily sending limit for the email account.

• Set the warm-up limit in the warm-up settings
• Set the sending limit per mailbox in your email account settings
• Set a campaign-level limit of emails each campaign can send

Recommended campaign and warm-up balance

Split sending between campaign and warm-up. For every campaign email, send 2 warm-up emails. Snovio recommends these limits:

Total daily sending (warm-up + campaign) is 50 emails/day or less per account.

Start with these limits, then gradually increase campaign sending.

• 30 warm-up emails/day
• 15 campaign emails/day

Each week, add 5 emails per day to campaign sending and reduce warm-up by 5 emails.

Once you reach 30 campaign emails and 15 warm-up emails per day, stop increasing the limits.

If you notice deliverability issues, reduce campaign sending and increase warm-up emails.

Schedule & sending hours

Change your campaign schedule a few times per month so it doesn't look identical over a long period.

Create schedule presets and reuse them across campaigns.

• Segment your prospect lists by location (country or region)
• Set up schedule presets with the correct time zone for each segment

• Match the prospect’s workday, not yours: schedule preset is in the prospect’s local hours

• Spread sending: 2-hour window in the morning + 2-hour window in the afternoon or evening

Tracking

Tracking can increase the chance of your emails landing in spam.

We recommend open and link tracking are disabled unless your sequences use behavioral triggers.

If tracking is required:

• Set up a custom tracking domain and add it to warm-up

Open rate

Opens are generally a positive reputation signal, and monitoring them can help you spot if your emails may be going to spam.

Remember that some opens are generated by bots and spam filters that scan your emails.

Open rate 50–70%: This is a good range. Even if some opens are from bots, it means a big part of prospects see and read your emails.

Open rate 30–50%: This result means your setup could use some optimization. Don’t increase sending limits yet.

Try these steps to troubleshoot:

• Run deliverability tests and fix any issues Snov.io detected
• A/B test your subject lines and content

Open rate 10–20%: This points to serious issues with spam. Your emails may not be reaching prospects. Most of these openings are coming from bots.

Don’t add more prospects to this campaign until you change your settings. Take these steps:

• Enable plain-text mode (removes images, links, and attachments)
• Disable link tracking unless you’re using a custom tracking domain
• Lower your campaign sending limits and add more to warm-up

Deliverability tests

Deliverability Check performs simulation sending and helps detect issues with technical setup or email content.

Run tests regularly and review your inbox vs. spam score. Since providers can treat the same email differently, check your real email’s deliverability by provider.

When to run tests

• After warm-up (Before launching any campaign)
• At least once a week while campaigns are active

Test results:

Check the results after running the tests.

Domain health

• 75% domain health is the minimum acceptable score to start campaigns; 100% score is recommended.

Inbox rate

• 90% inbox rate across providers (Gmail/Outlook): the sender can be used.
• 80% inbox rate: fix any underlying issues. Extend warm-up period or rebalance more volume from campaigns into warm-up.

Spam rate

• 5% or more: Don’t use this sender account for campaign yet → keep it on the warm-up only

IP address reputation

Check the health of the IP addresses you use to send emails.

Deliverability tests in Snov.io also include checking your IP Sender score and IP blacklist check.

Sender score (IP address reputation)

• The score should be above 70 or more for all IP addresses used

Blacklists

• There should be no blacklisting on active senders

Final checklist: How to avoid spam filters

Technical setup

• SPF, DKIM, DMARC, and MX are properly configured
• Use dedicated outreach domains aged 30+ days
• Set up Google Workspace + Outlook mailboxes (for provider matching)

Sender behavior

• Start with warm-up
• Use safe sending limits: up to 30 emails per day per mailbox
• Maintain a 5%+ reply rate
• Run a deliverability check once per week

Email content

• Keep it short: 50–100 words for the first email, 20–50 for follow-ups
• One clear CTA
• Plain text or minimal HTML
• Personalized subject line (the prospect or company name)

List quality

• Use valid, verified email addresses
• Segment lists by ICP
• Monitor list health and use yellow (risky) emails carefully
• Remove bounced contacts right away
• Filter your list by provider and use provider matching in campaigns

What’s the impact of spam filters

Spam filters learned to detect bulk senders and low-quality campaigns.

Reaching the inbox is harder than ever. But if you follow the right practices, you can stand out from other senders trying to reach the same leads.

Senders who follow the rules get higher visibility in recipients’ inboxes. Only a small number of senders do it consistently. Be one of them.