What Are MX Records?
MX stands for Mail Exchange. MX records are a type of DNS (Domain Name System) entry that tell email senders where to deliver email for a given domain.
For example:
example.com → MX → mailserver.example.comWithout a working MX record, your message has nowhere to go.
Think of MX records as the postal routing system for your emails—if there's no destination listed, your mail can't be delivered.
What Is an MX Check?
An MX check confirms that a domain has at least one working MX record configured in DNS.
Email validators like Lero run an MX check as part of their verification pipeline to determine whether:
- The domain is set up to receive email
- The record points to a real, functioning mail server
- There are obvious issues (e.g., typos, expired DNS)
MX checks are a non-invasive, fast way to filter out bad domains before attempting riskier checks like SMTP pings.
Why MX Checks Matter in Email Validation
MX checks are often the first filter in modern email validation. They matter because:
- Invalid domains = hard bounce risk
- No MX record = definitely undeliverable
- Temporary domains (like tempmail.net) often have suspect MXs
By checking MX records first, validators can reject invalid emails quickly, saving time and bandwidth.
📘 Dive deeper: Read our full Email Validation Guide ← (links to P1)
MX Check vs. SMTP Check
| Feature | MX Check | SMTP Check |
|---|---|---|
| What it checks | Domain-level ability to receive mail | Inbox-level mailbox existence |
| Risk level | Low (no connection) | Medium (can trigger server defenses) |
| Speed | Fast (DNS lookup) | Slower (~200–1000ms) |
| Usage scenario | Pre-filter to discard invalid domains | Final check for deliverability |
TL;DR:
Use MX checks to validate the domain, and SMTP checks to validate the mailbox.
How an MX Check Works (Step-by-Step)
Here's what happens during a typical MX check:
1. DNS Query Initiated
The validator asks a DNS resolver: "Who handles mail for domain.com?"
2. DNS Response Returned
The resolver replies with MX records—often multiple, with different priorities.
3. Priority & TTL Read
The system evaluates which mail server to try first and caches the result temporarily.
4. Result Interpreted
If no MX record is found, the domain is marked as invalid or non-functional.
Real-World Use Cases of MX Checks
MX checks aren't just for developers—they help across teams:
- Signup forms: Prevent users from submitting fake/invalid domains.
- Outbound email: Clean up lead lists before a cold campaign.
- CRMs: Catch expired or mistyped domains before a sequence begins.
- Lead scoring: Flag disposable or suspicious domains with weak MX setups.
How MX Records Affect Deliverability
If you're sending to domains without MX records or misconfigured entries:
- Your messages will bounce instantly (hard bounce)
- Your domain reputation may drop with major inbox providers
- ESPs may flag you as a low-quality sender
MX health is directly tied to email hygiene, which impacts both cost and conversion.
📬 Want the full picture? Read Email Deliverability 101 ← (links to P2)
Tools That Perform MX Checks
You can run MX checks using:
| Tool | Purpose |
|---|---|
| Lero | API-first validator with 97.3% accuracy |
| MXToolbox | Free web-based MX record tester |
| Dig/nslookup | Command-line DNS tools |
| Google Admin Toolbox | Good for GSuite-related domains |
For developers, using a validator with real-time MX lookups means less friction in workflows like onboarding or list syncing.
Role of MX Checks in Lero's Validation Pipeline
Lero's engine runs MX checks as one of the first and fastest steps in its email validation process.
Why?
- ⚡ Sub-50ms speed
- 🛡 No inbox triggering
- 🧠 Helps quickly separate "no-go" emails before costlier SMTP tests
It pairs MX checks with:
- Syntax validation
- Disposable detection
- SMTP ping (only if MX passes)
✅ Try Lero free with 500 validations → Start Now
Developer Tip: Checking MX Records via CLI or Code
Using dig (macOS/Linux):
dig mx example.comUsing nslookup (Windows):
nslookup -type=mx example.comPython Example:
import dns.resolver
answers = dns.resolver.resolve('example.com', 'MX')
for rdata in answers:
print(f'Priority: {rdata.preference}, Mailserver: {rdata.exchange}')These methods are great for building internal email tools or integrating checks into signup pipelines.
FAQs About MX Record Checks
Q1: Can an email domain work without MX records?
Technically no—without MX records, mail servers don't know where to deliver messages.
Q2: Does MX check validate the mailbox?
No—it checks if the domain can receive mail, not if the individual address exists.
Q3: How often do MX records change?
Rarely. Most domains keep their MX configuration stable, but DNS TTL may vary.
Q4: What happens if there are multiple MX records?
Mail servers use the one with the lowest priority number first.
Q5: Can a domain have no MX but still accept mail?
Fallbacks to A records are deprecated. Most modern servers reject such configurations.
Q6: Do free validators like Lero check MX records?
Yes—MX check is a core part of Lero's pipeline, done in real time.
Final Thoughts + Link to Broader Deliverability Guide
MX checks are essential, yet often overlooked, in email validation and deliverability strategies.
📚 Keep learning:
- • Read the Email Validation Guide ← (P1)
- • Explore Deliverability 101 ← (P2)