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Subdomain Best Practices for Cold Emailing with Mission Inbox

Using subdomains effectively is one of the most strategic moves you can make to improve deliverability and isolate risks when sending outbound emails. This guide covers how to choose, set up, and manage subdomains in Mission Inbox.

📍 What is a Subdomain?

A subdomain is a prefix to your root domain. For example:

  • Root domain: company.com
  • Subdomain: get.company.com

Even if you're purchasing secondary domains, it's still not recommended to send emails from your primary domain using subdomains. Instead, a better approach is to create three subdomains under your secondary (prospecting) domain and assign one mailbox per subdomain. This helps maintain deliverability, keeps each sending identity isolated, and allows you to scale without increasing your costs. Subdomains let you separate different email sending environments while maintaining brand consistency and deliverability.

🕵️ Why Use Subdomains?

  • Deliverability Isolation: Damage to your subdomain’s reputation won’t affect your main domain.
  • Scalability: Each subdomain can host its own mailboxes, allowing you to scale outreach.
  • Segmentation: Use different subdomains per brand, team, or campaign.
  • No Cost to You: Mission Inbox does not charge per domain or subdomain added, nor will your domain registrar as they are made from DNS records.

Note: Using a subdomain is safer than risking your primary brand domain (e.g., company.com) when doing high-volume cold outreach.

📝 Recommended Subdomain Names

Choose names that are short, clean, and non-promotional:

✅ Good Options:

❌ Avoid These:

  • mail.
  • email.
  • mx.
  • Long or keyword-heavy subdomains (e.g., outreach-email-sequence.company.com)

🛠️ How to Create Subdomains in Mission Inbox

  1. Go to the Domains tab inside your Mission Inbox account.
  2. Click Add Domain.
  3. Enter the full subdomain (e.g., get.company.com) manually or via the bulk upload option.
  4. Choose your DNS setup option:
    • API-based setup (GoDaddy/Cloudflare): DNS records are pushed automatically.
    • Manual DNS setup (Porkbun, Spaceship, etc.): You’ll be shown the records to copy into your registrar.
  5. Complete the DNS verification process.
  6. Once verified, you can begin creating mailboxes tied to that subdomain.

Pro Tip: Always verify the subdomain DNS before bulk uploading mailboxes.

🏠 Root Domain vs. Subdomain: Choose One

You must choose between using the root domain OR a subdomain for your mailboxes. Do not use both simultaneously. We recommend following a best practice setup: three subdomains per root domain, with one mailbox assigned to each subdomain. 

Why?

  • Mixed usage confuses spam filters
  • Sharing reputation between root and subdomain reduces the protective benefits
  • Compromises isolation and makes it harder to troubleshoot

⚠️ If you create mailboxes on company.com, do not create additional ones on get.company.com or vice versa.

📂 Bulk Uploading Mailboxes to Subdomains

Mission Inbox allows you to bulk upload mailboxes tied to subdomains:

  1. Use the provided CSV template
  2. Add mailboxes like inbox1@get.company.com, inbox2@get.company.com, etc.
  3. Upload to Mission Inbox

Ensure the subdomain is fully DNS-verified before starting the upload.

🌐 Should You Add the Root Domain Too?

Yes—but only for DNS purposes. Adding the root domain helps with authentication (e.g., DMARC alignment). However, you should not send from it if you are sending from a subdomain.