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 spam filters check

• Base requirements to avoid spam filters

• Advanced settings to improve deliverability

• Final checklist how to avoid 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 spam filters check

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:

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.

Sender infrastructure

Domains & mailboxes

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

Buy separate outreach domains and give them some time to age. Sending campaigns from a new domain can make it look spammy.

• Buy domains at least 30 days before use (even before warm-up). It’s better to send from aged domains
• 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

Technical 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.

Email accounts and 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 multiple inboxes (created across multiple domains 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. Don’t start sending campaigns until your domain is at least 30 days old.

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.

Enable plain-text mode for the first email in your sequence. Avoid too many design elements or images and use simple formatting in your emails.

Images/links/attachments

• No images, or links in the first email
• Don't include attachments (at least 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, use https:// (not http://) and show the full URL. Avoid hiding links behind CTA text like Click here.

Set up A/B tests for each email in a sequence. Test different subject lines or email content variations.

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.

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 replies, work on your ICP and define your strongest selling points.

Only contact decision-makers or leads 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.

Send campaigns to verified email addresses. Verify your email list before every campaign and check list health.

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

• Remove any bounced contacts from your campaign right away
• Add them to your Do-not-contact list

Advanced settings (good to have)

Following these recommendations will improve the performance of your sender accounts.

Provider matching

Segment prospects by provider to choose the best way to contact them. Emails sent within the same provider ecosystem often have better deliverability.

Connect mailboxes from different providers: optimal combo is 70% Google mailboxes / 30% Microsoft Outlook mailboxes. If you can only use one provider, we recommend Google Workspace.

Use the filter on the Prospects dashboard to see how many leads use Gmail, Outlook, and other providers. Enable Provider matching in campaigns.

Campaigns + warm-up (deliverability protection)

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

Use warm-up as a background process to protect deliverability. 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

Don't use full sending limit for outreach. Leave part of the account’s limit for warm-up. Warm-up and campaign limits are set separately, but both count toward the email account’s total daily sending limit.

• 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

Your daily sending limit should be split between campaign emails and warm-up emails.

Snov.io recommends keeping your total daily sending (campaign + warm-up) at 50 emails per day or less per account.

Start with:

• 30 warm-up emails per day

• 15 campaign emails per day

Then gradually shift volume from warm-up to campaigns until you reach 30 campaign and 15 warm-up per day. Each week, add 5 to campaign sending and reduce warm-up by 5.

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

Open tracking

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

On the downside, some opens are generated by bots and spam filters, and the tracking pixel can make your email look suspicious to spam filters.

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.

Deliverability tests

Perform simulation sending to detect issues with technical setup, email content, blacklists and sender IPs.

Run deliverability 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 deliverability 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.

• 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 → use Warm-up to restore its reputation.

Back-up domains

Pause sending from the domain with poor deliverability. Keep it on warm-up until its reputation improves (at least two weeks).

Set up additional domains and start warming them so you have backups. While your email domain recovers, use LinkedIn as an outreach channel.

IP address check

Check the reputation of the IP addresses used to send emails by your provider.

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 before increasing limits
• Run a deliverability check once per week

Email content

• Keep emails 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.