Cold Email MX Record Configuration: The Inbound Side of Deliverability

By Puzzle Inbox Team · Apr 24, 2026 · 6 min read

MX records tell the world where to deliver email sent to your cold email domain. Misconfigured MX records cause reply loss and deliverability problems.

What MX Records Do for Cold Email

MX records (Mail Exchange records) are DNS records that specify which mail servers are authorized to receive email for your domain. When someone replies to your cold email, their email server looks up your MX records to figure out where to deliver the reply. If your MX records are missing or misconfigured, replies silently fail — prospects who reply to your cold email see their responses bounce, and your team never knows it happened.

The Cold Email MX Record Nightmare

Here is a scenario I see constantly: cold email operator sends 10,000 emails, gets "only" 50 replies, declares the copy a failure, moves on. Reality: they got 120 replies. 70 of them bounced because their MX records were broken, so the prospect's reply never reached them and they never knew. The campaign was actually successful.

MX record configuration is the silent killer of cold email campaigns. And it is entirely preventable.

How to Configure MX Records for Google Workspace

For Google Workspace cold email inboxes, the MX records should point to Google mail servers. The standard configuration:

Priority 1: aspmx.l.google.com
Priority 5: alt1.aspmx.l.google.com
Priority 5: alt2.aspmx.l.google.com
Priority 10: alt3.aspmx.l.google.com
Priority 10: alt4.aspmx.l.google.com

Lower priority numbers take precedence. If Google primary servers are unavailable, mail flows to alternates automatically.

How to Configure MX Records for Microsoft 365

For Outlook 365 cold email inboxes:

Priority 0: yourdomain-com.mail.protection.outlook.com (your specific tenant domain)

Microsoft provides one primary MX server per tenant. The specific domain comes from your Microsoft 365 admin portal.

How to Verify MX Records

Use MXToolbox (mxtoolbox.com) MX lookup tool. Enter your cold email domain. You should see:

All expected MX records present
No errors or warnings
Reverse lookups resolving correctly
No conflicting records from previous configurations

If any of these fail, your replies may be dropping. Fix before scaling cold email volume.

Common MX Record Mistakes in Cold Email

No MX records at all: Domain registered, inbox configured, but DNS has no MX records. All replies bounce. This happens when domain registrar DNS defaults are wrong.
Conflicting MX records: Previous email provider MX records still in DNS alongside new ones. Causes intermittent delivery failures.
Wrong priority values: Priorities inverted, so backup servers get mail first. Technically works but produces slower delivery.
MX records pointing to A records: Some DNS providers silently convert MX records to A records, which breaks email routing entirely.
Typos in MX destinations: asmpx.l.google.com instead of aspmx.l.google.com. One letter off = total failure.

MX Records and Cold Email Providers

Quality cold email infrastructure providers like Puzzle Inbox configure MX records correctly as part of inbox provisioning. Budget providers often leave MX configuration to the customer, which is where most MX mistakes happen. If you are self-configuring cold email domains, verify MX records through MXToolbox within 24 hours of setup — before you start sending campaigns that produce replies you will lose.

Subdomain MX Records

If you use a sending domain like mail.yourdomain.com (subdomain), the MX records must be configured on the subdomain specifically. Parent domain MX records do not automatically apply to subdomains. This is a common error in sophisticated cold email setups using subdomain strategies.

Verify MX records within 24 hours of setting up any cold email domain. Bad MX records cost you replies silently — the most expensive kind of deliverability problem because you never see it happening. Use MXToolbox, fix any errors, then start sending.