What Is Email Syntax?
Email syntax refers to the structure and format of an email address. It must follow a specific pattern to be accepted by servers—and even minor errors can cause bounces or validation failures.
Syntax ≠ Deliverability
Just because an email address looks right doesn't mean it:
- Actually exists
- Can receive email
- Won't bounce
But syntax is your first line of defense in catching obvious issues early—especially during signups or imports.
What Does "RFC-Compliant" Mean in Email Syntax?
RFCs (Request for Comments) are technical standards that define how internet protocols work.
Two main RFCs govern email syntax:
- RFC 5321: Email transport rules (SMTP)
- RFC 5322: Email header and format rules
To be RFC-compliant, an address must follow these specifications:
- One @ symbol only
- No special characters outside quoted strings
- Valid domain and TLD
- Total address ≤ 254 characters
✅ Valid Address:
"jane.doe"@example.com❌ Broken Address:
jane@@example..comEven a double dot breaks RFC syntax.
Common Mistakes in Email Syntax
Here are the most common violations caught by validators like Lero:
| ❌ Invalid Syntax | ✅ Fix |
|---|---|
| john..smith@example.com | john.smith@example.com |
| @gmail.com | john@gmail.com |
| alice@company | alice@company.com |
| foo@.com | foo@example.com |
| sam@ex ample.com | sam@example.com |
Even though many of these will pass through casual regex, they fail at delivery time.
Anatomy of a Valid Email Address
Let's break it down:
username@domain.tldRules:
Username (local part):
- Up to 64 characters
- Letters, numbers, dots, underscores, and dashes
- Special characters (+, =, etc.) allowed in quotes
Domain:
- Up to 253 characters total
- Must include at least one dot (.)
- No special characters or consecutive hyphens
Full length:
- Must be ≤ 254 characters
Email Regex Patterns (Explained Simply)
Regex (Regular Expressions) are used by validators to programmatically match valid email patterns.
Common Regex:
^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$But this simple regex:
- Misses edge cases
- Fails on quoted local parts
- Allows double dots
What Lero Uses:
Lero uses a layered regex engine, balancing RFC-accuracy and speed. It's designed to reject "borderline" syntaxes that may pass but still bounce.
When Syntax Alone Isn't Enough
An address may be syntactically valid, but:
- The domain doesn't have MX records
- The inbox doesn't exist
- It's a spam trap or disposable inbox
That's why syntax validation is only step 1.
📬 To go deeper, check our Email Validation Guide ← (P1 link)
How Validators Check Syntax Under the Hood
Validators follow this flow:
Lero's syntax layer is sub-10ms, meaning your signup flow or bulk import isn't slowed down.
Free PDF: The Email Syntax Checklist
Looking for a ready-to-go syntax quality guide?
🎁 Download our Free Email Syntax Checklist (PDF)
→ Download PDFWhat's Inside:
You can hand this off to your:
- Growth team
- Developers
- SDRs managing outbound lists
How Lero Uses Syntax Checks in Validation
Lero runs syntax checks before hitting SMTP or MX layers.
Why:
- Catches 60–80% of bad addresses instantly
- Avoids unnecessary server queries
- Tags each invalid result with precise error messages
This makes Lero's output actionable—not just "valid" or "invalid."
💡 Learn more: See the full Lero tech breakdown ← (P1 link)
Why Syntax Hygiene Affects Deliverability
Poor syntax = more bounces = worse sender reputation.
If 5% of your emails bounce due to typos or formatting errors:
- Your ESP may throttle you
- ISPs may flag you as a spammer
- Inbox placement suffers—even for good leads
💡 Dive into Deliverability 101 to fix that ← (P2 link)
FAQs About Email Syntax Validation
Q1: What's the longest an email can be?
254 characters total (64 for local part, 190+ for domain/TLD)
Q2: Can an email have two @ symbols?
No. Only one @ is allowed.
Q3: Can emails include emojis or special characters?
Only under strict conditions (UTF-8 + quoted strings)—best to avoid.
Q4: Does passing regex mean the email is safe?
Not always. Use SMTP and MX checks to confirm the address works.
Q5: Can syntax errors cause spam issues?
Yes—especially if you send to typo'd or malformed addresses repeatedly.
Q6: Is your checklist developer-friendly?
Yes. Regex and error codes are included in the PDF.
Final Tips & What to Do Next
If your forms or CRM are allowing bad emails in, syntax validation is step one to fix that.
✅ Remember:
- Syntax ≠ existence
- Regex ≠ RFC-compliance
- PDF checklist = productivity boost
📥 Next Steps:
- • Get your Free Syntax Checklist PDF
- • Read the full Email Validation Guide (P1)
- • Level up with Deliverability 101 (P2)