Scaling Without Breaking Deliverability

Mailbox providers such as Gmail and Outlook judge you partly by how much you send and how steady it is. A sudden jump in volume looks like a spammer, even when every email is wanted. This playbook shows you how to grow your sending step by step: set up your domains and IPs, warm them up, and add volume without losing the inbox.

Why sending more is risky

Real businesses tend to grow slowly. Spammers tend to appear from nowhere and send a lot at once. So when your volume jumps, providers get suspicious. A shop that goes from 10,000 emails a day to 100,000 overnight can expect:

• Throttling: the provider accepts only a few emails at a time and asks you to slow down.
• Deferrals: the provider says “try again later” (a temporary error, with a code starting with 4).
• Spam placement: the mail is accepted but goes to the spam folder.
• Blocks: the provider refuses the mail outright (a permanent error, with a code starting with 5).

This happens even if every new recipient asked for your email.

Plan your setup

Shared or dedicated IP

Every email leaves from an IP address (the numeric address of the sending server). Providers track a reputation for each one.

• Shared IP. Your email platform sends for you and many other customers from the same IPs. This suits most senders, but other customers’ behaviour affects you, and your platform may limit how fast you grow.
• Dedicated IP. An IP only you use. Nobody else can harm its reputation, and nobody else’s good reputation helps you. It only makes sense for a large, steady daily volume. An IP that sends little mail can’t build a reputation.
• Several dedicated IPs. At very high volume, you can split traffic across IPs and manage each kind of email on its own.

Keep each kind of email separate

Receipts and password resets must always arrive. Newsletters are more likely to draw spam complaints. If both share one reputation, a bad newsletter can delay a password reset. So give each kind of email (each stream) its own subdomain, and at high volume its own IPs.

A setup for shop.example might look like this:

shop.example          website and staff email, no bulk sending
├── mail.shop.example   receipts, order updates, password resets
├── news.shop.example   newsletters and offers
└── notify.shop.example automatic reminders and alerts

Each subdomain builds its own reputation. Providers still link it to your main domain, so the protection is partial.

Each sending subdomain needs:

• SPF (a DNS record listing the servers allowed to send for it), covering only its own senders
• DKIM (a digital signature on each email), with its own key
• DMARC (a DNS record telling receivers what to do with mail that fails the checks). One record on the main domain can cover all subdomains.
• an address that can receive bounces and replies

If you run your own servers, each IP also needs reverse DNS (a record that maps the IP back to a hostname). Check each subdomain with Mailwel’s DNS lookup and test inbox.

Warm up new IPs and domains

A new IP

A new IP has no reputation. Warming up means sending small amounts at first and growing slowly, so providers can see that people want your mail. Here is an example schedule for a dedicated IP. If your email platform gives you its own, follow theirs.

Week	Emails per day	Who to send to
1	500–1,000	Your most engaged subscribers (opened in the last 30 days)
2	1,000–2,500	Still your most engaged
3	2,500–5,000	Opened in the last 60 days
4	5,000–10,000	Opened in the last 90 days
5	10,000–25,000	Opened in the last 120 days
6	25,000–50,000	Opened in the last 180 days
7	50,000–100,000	Your full active list
8 onwards	100,000+	Normal sending

While you warm up:

• Start with your most engaged readers. They open, click and rarely complain.
• Don’t skip weeks. Each week builds on the last.
• Spread mail across providers, not all to Gmail.
• Send real emails every day. Providers can spot test messages, and gaps don’t build trust.

A new domain or subdomain

A new domain starts with no reputation, so filters are unsure about you. Expect some spam placement in the first few weeks. Passing authentication on every email, and sending to people who engage, builds trust faster.

Check these every day while you warm up

• Over 98% of your mail is accepted at each provider
• Under 1% of your mail bounces
• You’re on no new blocklists (public lists of spam sources)
• Google Postmaster Tools shows no rise in your spam rate
• No permanent rejections (codes starting with 5) for policy or spam reasons
• Under 5% of your mail is deferred (codes starting with 4)
• Spam complaints stay very low, well under 0.1%

If any check fails, stop increasing and hold your volume until it recovers. If things get worse, send less.

Grow an established program

Even with a good reputation, growth needs care.

Never more than double in a week

A common rule of thumb: never more than double your volume from one week to the next. Smaller steps are safer. This example grows from 50,000 to 500,000 emails a week:

Week	Emails that week	Increase
1	50,000	Starting point
2	80,000	+60%
3	120,000	+50%
4	180,000	+50%
5	270,000	+50%
6	400,000	+48%
7	500,000	+25% (target reached)

Spread the extra volume

• Across providers, in line with where your subscribers are. If 40% use Gmail, about 40% of the new mail goes to Gmail.
• Across the day. Send over several hours. Providers limit how much they accept per hour.
• Across the week. Send a second weekly email on a different day from the first.

Adding new subscribers

When growth comes from new people, such as a new signup source or an old list:

1. Check the addresses first with an email validation service. Remove invalid, throwaway and role addresses (like info@ or sales@).
2. Mix them in with your engaged readers, so good engagement balances out bad signals.
3. Track them separately. If the new group draws bounces or complaints, pause it without hurting the rest of your list.

Provider differences

Gmail

• Watch your spam rate and delivery errors in Google Postmaster Tools as you grow.
• Gmail doesn’t publish its limits. A 421 error about rate limits or unusual volume means “slow down”.
• If you send 5,000 or more emails a day to Gmail users, you must meet its bulk sender rules: SPF, DKIM, DMARC, one-click unsubscribe, and a spam rate under 0.3%. Yahoo has the same rules.

Microsoft

• Outlook.com pays close attention to IP reputation. New IPs get lower limits than trusted ones.
• Register your IPs in Microsoft SNDS (Smart Network Data Services) to see how Outlook.com rates them.
• Warming up with Microsoft often takes longer than with Gmail.
• Microsoft has required SPF, DKIM and DMARC for bulk senders to Outlook.com since May 2025.

Yahoo and AOL

• Yahoo limits how many connections a new or unknown IP can open at once. Send slowly to Yahoo at first.
• Yahoo reacts strongly to spam complaints. Even a modest complaint rate can lead to blocks.

Automated emails grow differently

Triggered emails are sent when someone does something, like placing an order. Their volume grows with your users, gradually, and each email is expected. Providers usually accept this growth well.

Drip sequences are a series of emails sent over days or weeks after someone joins, such as a welcome series. They also grow with signups. Watch out for:

• Long gaps between emails. People forget they signed up, and report you as spam.
• Big imports. Adding 50,000 contacts to a sequence on one day is a sudden spike.
• Old content. Review and update your sequences regularly.

Checklist

Setup

• Choose a shared or dedicated IP based on your steady daily volume
• Write down a warm-up plan for each new IP
• Give each stream (receipts, newsletters, alerts) its own subdomain
• Set up SPF, DKIM and DMARC for every sending subdomain, and test each in the test inbox
• Set up reverse DNS for any IP you run yourself, and speed limits per provider

List quality

• Validate all new addresses before you send
• Remove addresses that hard bounce (permanently fail)
• Remove or separate people who haven’t engaged in about six months
• Sign up for feedback loops (complaint reports) at Microsoft and Yahoo
• Remove anyone who complains, automatically and at once

Monitoring

• Verify your domain in Google Postmaster Tools
• Register your sending IPs in Microsoft SNDS
• Check blocklists automatically, and set alerts for bounces, complaints and listings
• Run seed tests and review the numbers daily during warm-up

Content

• Send real, relevant emails, not test messages
• Include one-click unsubscribe in every marketing email
• Include your postal address and who you are
• Keep the same From name and address across sends