Increasing sending limits with multiple domains

To send large volumes of email, you might need more than one sending domain. Every domain has a daily sending limit that grows only as the domain builds a good reputation, and transactional emails should always travel from a separate domain so a marketing problem can never block them. This article explains why, and how to set it up.

Why sending limits exist

Sending limits exist because mailbox providers judge every sending domain by its reputation, and a new domain has none. A sudden burst of thousands of emails from a domain nobody has seen before looks exactly like spam, so providers react by filtering or rejecting it.

To protect your inbox placement, Sendigram caps how many emails a domain can send per day. The cap is usually low for a new domain and rises as the domain proves it is a trustworthy sender. A limit is not a penalty. It keeps you from damaging a domain before it has had a chance to build trust.

Mailbox providers look at several signals when they decide how to treat your emails:

• Domain and IP reputation: your history of sending wanted, low-complaint email.
• Engagement: how many recipients open, click, reply, or move your emails out of spam.
• Complaints: how often recipients press the spam button.
• Bounces: how many addresses do not exist, which suggests an unclean or purchased list.
• Consistency: steady volumes build trust, while sudden spikes raise suspicion.
• Authentication: whether SPF, DKIM and DMARC are set up correctly for the domain.

Why more volume means more domains

A single domain can only safely send as much as its reputation allows, so the way to send more per day is to add more warmed-up domains and share the volume between them. Pushing one domain past its limit puts all of your sending at risk.

For illustration only: if one warmed-up domain comfortably handles 50,000 emails a day, a goal of 200,000 emails a day needs about four warmed-up domains, each carrying a quarter of the volume.

What multiple domains give you:

• Capacity: each domain has its own daily limit, so total volume grows with the number of domains.
• Risk isolation: if one domain has a bad campaign, the others keep their reputation and your sending does not stop.
• Control: you can assign domains to different brands, regions, or audience segments, and watch each reputation separately.

Things to keep in mind:

• Each domain needs its own warm-up. A new domain does not inherit the reputation of your main domain.
• Split volume evenly and keep it steady. Do not switch heavy traffic between domains day by day to avoid limits.
• Subdomains and separate domains differ. A subdomain such as news.example.com has its own reputation, but providers may also take the main domain into account. For full isolation, use a separate domain.
• Each domain needs its own SPF, DKIM and DMARC records.

Why transactional emails need their own domain

Transactional emails should be sent from a separate domain so that a problem with a marketing campaign can never block the messages your customers depend on. This is not strictly enforced, but in practice it is almost a must.

Transactional emails are triggered by a person's action and carry information they expect right away. Typical examples are:

• Password reset links and login codes
• Order confirmations, receipts and invoices
• Shipping and delivery updates
• Account, security and billing alerts

Marketing campaigns carry more risk. They go to large lists, they are not requested at the moment of sending, and they get more spam complaints and unsubscribes. If marketing and transactional emails share one domain, they also share one reputation. One badly received campaign can push the whole domain into spam folders or get it blocked, and your password resets and receipts go down with it.

Why a separate domain protects you:

• Critical emails keep flowing. A customer who cannot receive a login code cannot use your product, and a missing receipt creates support tickets and lost trust.
• Reputation stays clean. Transactional emails usually get high engagement and few complaints, which builds a strong reputation. That reputation should not be dragged down by promotions.
• Problems are easier to find. With separate domains, a drop in delivery clearly points to either marketing or transactional traffic.
• Recovery is faster. If the marketing domain is damaged, you can pause it and repair it while customer-facing emails continue untouched.

Good practice for the transactional domain:

• Use a clearly related name, for example notify.example.com or account.example.com, with marketing on something like news.example.com.
• Warm it up too, and keep its sending steady, even though its volume is often smaller.
• Authenticate it with SPF, DKIM and DMARC, and use its own tracking and bounce domain.
• Keep promotional content out of transactional emails. Mixing offers into receipts or password resets can hurt reputation, and may change how the message is treated under email marketing laws.