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Email Deliverability Best Practices

If you’re using AI to scale your outbound you’re likely asking the same question we hear all the time:

“How many emails can I safely send per SDR without damaging deliverability?”

It’s a critical question. AI helps teams move faster and reach more prospects, but over-automation can backfire quickly. Spam filters are getting smarter, major email providers (like Google and Office 365) are cracking down on bulk sends, and sending too many emails that receive spam complaints can tank your sender reputation.

Over the past year, we’ve tested and tracked what works (and what doesn’t) when it comes to high-volume, AI-generated outbound. Here’s what we’ve learned about how you can stay on the right side of deliverability while still scaling your outreach.

The Short Answer: Cap AI-Sent Emails at 50-150 Per Mailbox Per Day

This recommendation comes from real-world testing and platform guidance (like Outreach). Sending too many emails per mailbox per day starts to trigger red flags for email providers and can lead to throttling or spam filtering, even if your content is solid.

Capping sends between 50-150/day keeps your email behavior more “human-like” and your domain reputation healthy.

So how do you scale sends? Using multiple domains and multiple mailboxes per outbound rep. At UserGems, each rep has 3-5 sending mailboxes that we cycle through. This spreads send volume across multiple inboxes so none of them are a single source of sends, and it mimics more natural sending patterns that inbox providers like to see. Many sales engagement platforms support some form of inbox rotation. Instantly and Smartlead automatically rotate through all mailboxes in a campaign. Outreach lets you assign multiple mailboxes per user and switch senders mid-sequence. Check your platform’s documentation for setup details.

UserGems uses Mailreach on every sending inbox to (a) keep them warmed with ongoing positive engagement and (b) give us a centralized dashboard to monitor each inbox’s health score. Other warmup tools include Mailreach, Instantly (built in), Smartlead (built in), Warmy, Warmup Inbox, Lemwarm, InboxAlly, and Maxify.co

Email Deliverability’s Evolution

A lot has evolved around email deliverability and even the best sales and marketing teams have seen their reply rates decline. That’s a reflection of changes in how email providers measure engagement and flag risk.

Some key trends:

  • Inbox providers weigh engagement heavily. The most important engagement factor for ISPs is spam complaints. Opens, replies, and even deletions can also have a (smaller) impact on your sender reputation. The goal is as few spam complaints per domain as possible, which is achieved both by spreading out send volume across mailboxes/domains and providing an easy unsubscribe method for recipients (source: Outreach). More on both below.
  • Repetitive AI-generated emails are easy to detect. High-volume, low-variation content is a spam trigger.
  • Large volume sends to cold or non-ICP contacts can hurt more than help. Low engagement rates drag down future inbox placement.

If you’re seeing fewer replies, it’s may not be a tools problem (assuming you’re following best practices like setting a daily mailbox cap and enabling easy unsubscribe). It’s a sending behavior problem.

11 Deliverability-Safe Practices for AI Email Outreach

1. DO NOT USE YOUR CORE DOMAIN FOR SENDING

“Don’t scream at me”. Sorry, but this is super important. Don’t use your core domain for sending (e.g. usergems.com). Once a domain is flagged it’s really hard to get your reputation back and the result is most/all of your emails landing in spam.

To keep your sending reputation healthy, set up multiple top-level domains (e.g. yourdomain.org) and create multiple email inboxes for each outbound rep under these domains. Note: If you have a technical resource who knows about DNS configuration, you can set up sub-domains (eg mail.yourdomain.com) instead of top-level. Sub-domains require more technical setup, so we suggest top-level domains if you’re just getting started.

2. Use multiple mailboxes per sender

For each outbound rep, create multiple mailboxes across several domains/sub-domains and use these for sending. At UserGems we have 3-5 mailboxes per rep based on our send volume. If you set mailboxes up using Google Workspace, you can generate an email forward to the outbound rep’s real inbox so they don’t have to log in to these extra mailboxes and simply receive replies to AI outbound emails in their main inbox.

3. Warm up your mailboxes and keep them warm

New mailboxes have no sending history, which means no reputation. If you start sending outbound at volume from a brand new inbox, email providers like Google and Microsoft will treat that activity as suspicious and you’ll land in spam.

Email warmup tools solve this by sending emails from your mailbox into a network of real inboxes (typically tens of thousands of Google Workspace and Office 365 accounts). Those inboxes then open your emails, reply to them, and mark them as “not spam” if they get filtered. This simulates the kind of positive engagement that builds sender reputation over time (source: Mailreach, source: Instantly).

How to warm up a new mailbox:

  • After creating a new mailbox, connect it to your warmup tool right away but don’t send any outbound for at least 2 weeks. Most tools start with just a few warmup emails per day and gradually ramp to 40-50/day during this period.
  • Before you start sending outbound, check your warmup tool’s health score or inbox placement rate. You want to see inbox placement above 90% before adding the mailbox to live campaigns.
  • Once warmed up, keep the warmup service running permanently. Warmup isn’t a one-time setup. Your outbound emails will naturally have lower engagement than regular email, and ongoing warmup compensates for that by generating consistent positive signals (source: Instantly).
  • Monitor inbox placement weekly. If a mailbox’s health drops, pull it from campaigns and let warmup recover it before sending again.

Popular warmup tools include Mailreach, Instantly (built in), Smartlead (built in), Warmy, Warmup Inbox, Lemwarm, InboxAlly, and Maxify.co. 

4. Send Fewer, Smarter Emails

More volume doesn’t mean more replies. Focus outbound on high-fit ICP accounts and prioritize prospects who’ve shown some type of buying signal or interest. Avoid spraying entire cold lists, especially non-ICP audiences.

How UserGems helps: This is core to what we do. UserGems’ Intelligence Agents analyze 600+ signals to build a custom scoring model based on your company’s sales history, not generic industry averages. Your team starts the day with a prioritized list of accounts and contacts that are most likely to buy right now so instead of blasting your entire TAM, you’re sending to the people who are most likely to engage, which directly protects your deliverability.

5. Limit Sends Per Mailbox Per Day

Limiting daily send volume mimics human behavior and prevents deliverability issues tied to volume spikes. Use multiple mailboxes per rep to scale sends while keeping each individual inbox healthy.

Tip: Here at UserGems, we use limits within our SEP to set firm daily mailbox maximums like 100 sends per mailbox per day.

6. Avoid Repetitive Subject Lines and Messaging

Too many AI emails use the same subject line and/or message structure. This is a red flag for inbox providers.

How UserGems helps: Gem-E’s personalization agent doesn’t just swap in a first name. It researches each prospect using signals and CRM data, then drafts genuinely unique emails grounded in real context. Because the personalization is signal-based (not template-based), each email is meaningfully different, which is exactly what inbox providers want to see. Gem-E sequences are seeing 6-20% reply rates vs. 1-2% industry average. That engagement is one of the strongest deliverability signals you can generate.

7. Limit Sequence Length for Low-Priority Audiences

If you’re emailing someone who hasn’t engaged and/or isn’t in your core ICP, keep it short. Three to four emails max. Long sequences to disengaged prospects increase negative sentiment signals (deletes, unsubscribes, ignores, or spam reports). As Outreach puts it: “persistent messages to users that are not engaged will damage reputation exponentially.”

How UserGems helps: Because UserGems scores every contact and account with a custom AI model based around your current customers, you can automatically route high-scoring prospects into longer, more varied sequences and low-scoring ones into shorter versions. The orchestration agent handles this routing for you, so your reps don’t have to make sequence-length decisions manually.

8. Clean and Validate Your Lists

Emails sourced through verified data partners (versus scraped or third-party lists) perform better. Make sure your emails are validated and your data sources are reliable. Never use purchased lists, which often contain spam traps that can land your domain on a blacklist (source: Outreach Deliverability Playbook).

Here are the bounce thresholds to know (source: Outreach Deliverability Playbook): Above 5%, use a validation service before sending. Above 10%, your ISP will start blocking your emails entirely.

At UserGems, we built an automated trigger that marks prospects as “bad contact info” when an email bounces and auto-finishes their sequences. This prevents bounced contacts from being re-sequenced and compounding the damage.

How UserGems helps: UserGems’ Data Agents continuously verify, enrich, and deduplicate your CRM data. When contacts change jobs, get new email addresses, or leave a company, UserGems catches it. This real-time data hygiene means you’re not sending to stale addresses that generate bounces and damage your reputation. We also use a validation service before providing an email to you, decreasing bounce rate.

9. Monitor Sender Reputation Regularly

Use tools to track how ISPs view your domain. Even a small increase in spam complaints can drag down inbox placement across the board. Here’s what to monitor:

  • Your SEP’s analytics: track reply rates, bounce rates, and unsubscribes at the sequence and inbox level. Look for trends, not snapshots.
  • Warmup tool dashboard (e.g., Mailreach, Warmy, Warmup Inbox, Lemwarm, InboxAlly, Maxify.co): check inbox health weekly for each sending address.
  • Google Postmaster Tools: free tool that shows how Google sees your domain reputation and spam rate. Note: Only relevant if you’re sending to gmail.com users.
  • Blacklist monitoring: tools like MxToolbox alert you if your domains or IPs land on a blacklist.

One note on open rates: open data is increasingly unreliable due to privacy features. Don’t track precise open percentages, but do pay attention if a campaign has a near-zero open rate, as that usually signals a bigger deliverability problem.

10. Make It Easy to Unsubscribe

While it might seem like unsubscribes are a bad thing, reframe them as providing a safe way out for recipients that keeps your domain healthy vs. forcing them to mark your message as spam to stop sends. An unsubscribe doesn’t hurt your sender reputation. A spam complaint does, significantly. Google now requires a one-click unsubscribe option for bulk senders.

11. Turn Off Open and Click Tracking for Cold Outbound

Tracking works by routing links through a tracking domain, which can trigger spam filters, especially if you’re on a shared tracking pool where your reputation is entangled with other senders. We’ve disabled both open and click tracking for our cold outbound sequences based on recommendations from Outreach.

Make Sure Your Infrastructure Is Set Up

Before any of the above matters, make sure the basics are in place. If your email authentication isn’t configured correctly, your emails may not make it out the door at all.

  • SPF (Sender Policy Framework): tells inbox providers which servers are allowed to send email from your domain.
  • DKIM (DomainKeys Identified Mail): adds a digital signature proving your emails haven’t been tampered with.
  • DMARC (Domain-based Message Authentication, Reporting and Conformance): ties SPF and DKIM together and tells providers what to do when authentication fails.

All three should be set up on every domain you send from. Your IT team can handle this. Outreach’s Email Deliverability Playbook walks through the details.

Also: use secondary domains for cold outbound (not your primary company domain), and make sure any new domains are warmed up for at least 2-4 weeks before sending at volume.

Bottom Line: Quality and Consistency Beat Volume

AI gives teams the ability to scale faster than ever. But scaling outbound shouldn’t mean risking your sender reputation. The teams winning with AI outbound today are sending smarter, not just more.

By capping sends, increasing personalization, and segmenting by audience fit, you’ll see better engagement and preserve the integrity of your domain. The best part: if you’re using UserGems, many of the hardest parts of this equation (targeting the right people, personalizing at scale, keeping your data clean) are already handled for you.

Further Reading

Questions about how this applies to your setup? Reach out to your UserGems CSM. We’re happy to talk through it.

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