Protecting your sending domain: warmup, limits and bounces

What sending reputation is

Mailbox providers decide whether to deliver, filter or block your email partly based on the reputation of your domain and the mailbox sending it. That reputation is built from signals such as whether your email is authenticated, how many messages bounce, how many recipients mark you as spam, and how people engage.

Reputation is slow to build and quick to damage. Because cold outreach goes to people who have never heard from you, it pays to be conservative.

Authenticate your domain

Authentication proves that email claiming to come from your domain really does. Set these up in your domain’s DNS, usually with instructions from your email provider.

1. 01

   Step 1: SPF

   Publish an SPF record listing the services allowed to send email for your domain. Keep a single SPF record per domain.

2. 02

   Step 2: DKIM

   Turn on DKIM signing in your email provider and publish the public key it gives you. This lets receivers verify the message was not altered.

3. 03

   Step 3: DMARC

   Publish a DMARC record. Many senders start with a monitoring policy (p=none) and a reporting address, then tighten the policy once they are sure all legitimate mail passes.

4. 04

   Step 4: Check it works

   Send a test to a mailbox you control and inspect the headers, or use your provider’s tools, to confirm SPF, DKIM and DMARC pass.

Warm up and keep daily limits

A new mailbox, or one that has never sent outreach, should start small. Send a handful of real, relevant emails a day, then increase gradually over several weeks while watching bounces and replies. Sudden jumps in volume are a common trigger for filtering.

Spread sends through the day rather than in one burst, and keep a daily ceiling even once you are established. Mailbox providers also set their own sending limits per account, which you should stay well under.

List hygiene and bounces

A hard bounce means the address does not exist or will never accept your mail. A soft bounce is temporary, such as a full mailbox. High bounce rates are a strong negative signal.

• Use contact details the business publishes, and avoid guessed or bought addresses.
• Remove hard bounces immediately and never send to them again.
• If an address soft-bounces repeatedly, stop sending to it.
• Honour opt-outs at once and keep a suppression list across campaigns.
• Watch your spam complaint rate. Google’s sender guidelines, for example, ask senders to keep reported spam rates low and to avoid ever reaching 0.3%.

Signs your reputation is slipping

Reputation problems rarely announce themselves. Watch for these patterns, and if you see them, pause new outreach, confirm authentication still passes, review where recent addresses came from, and restart at a lower volume.

• More bounces than usual from addresses that should be valid.
• Replies drop sharply even though your list and message have not changed.
• Test emails to your own accounts at other providers start landing in spam.
• Your email provider warns you about sending limits or unusual activity.

Should you use a separate domain?

Some teams send outreach from a secondary domain so that any reputation damage does not affect their main company email. Others prefer their primary domain because it is the most trustworthy to recipients. Either way, the same rules apply: authenticate it, warm it up, and keep volume and bounces low.