SMTP Error 555 5.5.2 – Syntax Error: Causes & Fixes

What is SMTP Error 555 5.5.2?

SMTP Error 555 5.5.2 is a syntax error that occurs during the email delivery process when a command, typically MAIL FROM or RCPT TO, is not properly formatted. It’s a subset of SMTP Error 555.

This error informs the sending server that the receiving mail server is unable to parse the command due to syntax error—often a malformed email address or some required formatting elements like angle brackets missing. “555” indicates a permanent failure, and “5.5.2” specifically means a syntax error in the SMTP conversation.

From the viewpoint of a mail server, this error is a defensive tactic. It keeps malformed messages from entering the system, thereby assisting in guarding against spam, spoofing, and abuse. The receiving server expects specific SMTP command formats and will immediately reject anything that deviates from the standard. So when it throws a 555 5.5.2 error, it’s essentially saying: “I don’t understand what you’re trying to send me—fix your syntax and try again.”

How email commands work (MAIL FROM & RCPT TO)

Basic SMTP command flow

SMTP (Simple Mail Transfer Protocol) is the foundation of how emails are sent across the internet. It operates through a series of commands exchanged between the sending and receiving servers. MAIL FROM (which defines the sender’s address) and RCPT TO (which defines the recipient’s address) are among the first and most important commands. These commands start the transaction and tell the receiving mail server who the message is from and who it’s going to.

A typical SMTP session might look like this:

EHLO sendingserver.com MAIL FROM:<[email protected]> RCPT TO:<[email protected]> DATA …

Each command must follow strict formatting rules for the session to continue without errors.

Just how important is correct syntax?

SMTP has very rigid syntax. Mail servers are not flexible. Even a single character, bracket, or space mistake can lead the server to drop the message. For example, the MAIL FROM command must include the email address enclosed in angle brackets (< and >). Skipping this formatting or using an invalid email string may lead to a 555 5.5.2 error.

Servers don’t attempt to “guess” what you meant—they respond with a hard error, which effectively blocks your message until the syntax is corrected. This is why ensuring command accuracy is essential to keeping your emails deliverable. 

Common causes of SMTP 555 5.5.2 – Syntax Error

SMTP Error 555 5.5.2 almost always boils down to the same thing: incorrect formatting. It’s a malformed email address, or a missing bracket, or some unexpected character in an SMTP command, and the receiving server is unable to process the instruction, so it throws a syntax error. While it may seem like a small detail, these formatting issues can completely block your email delivery. Below are the most frequent culprits behind this error.

Malformed email addresses

One of the most common reasons for a 555 5.5.2 error is a malformed email address. SMTP servers expect email addresses to follow a very strict structure: a local part, the @ symbol, and a valid domain name. Mistakes such as missing the @, using unsupported characters, or including typos in the domain can all result in syntax failures during the SMTP session.

For example:

• john.doe@ (missing domain)
• john@@example.com (multiple @ symbols)
  john.doe@exam!ple.com (invalid character)
• john.doe@example,com (comma instead of period)

These may seem like small errors, but they break the protocol rules and can trigger a hard SMTP rejection from the receiving mail server.

Missing angle brackets in SMTP commands

In the context of SMTP, angle brackets (< >) are not optional syntax—they’re required. The MAIL FROM and RCPT TO commands must wrap the email addresses in brackets to be valid. Without them, the receiving server may not interpret the address correctly, resulting in a 555 5.5.2 error.

Consider this correct version:

MAIL FROM:<[email protected]> RCPT TO:<[email protected]>

Now compare that to the following incorrect commands:

MAIL FROM:[email protected] ← missing brackets RCPT TO:<recipient@example,com> ← incorrect domain character MAIL FROM:<senderexample.com> ← missing “@” entirely

This small syntax requirement is often overlooked, especially in custom-built scripts or integrations.

Misconfigured email clients or scripts

Custom applications or poorly configured email clients are another major source of syntax errors. Developers often build SMTP support into software using frameworks or libraries in languages like Python, PHP, or Node.js. If the configuration isn’t handled correctly—or if raw email addresses are injected without validation—it can lead to malformed commands being sent during the handshake.

Some email clients may also strip required formatting or attempt to interpret it in non-standard ways. While major platforms are usually compliant, legacy or open-source clients may not catch these issues before sending.

Improper use of SMTP extensions

Modern SMTP servers frequently employ Extended SMTP (ESMTP), which adds support for new features such as authentication, pipelining, and 8-bit MIME. But not all servers support all extensions, and an unsupported or poorly formatted ESMTP command can also produce a syntax error.

For instance, sending commands like:

MAIL FROM:<[email protected]> SIZE=102400

…may cause issues if the receiving server doesn’t support the SIZE extension, or if the syntax doesn’t match its expectations.

Another common issue is declaring ESMTP capabilities during the handshake (EHLO) but then falling back to legacy SMTP syntax without consistency. This mismatch can confuse servers and lead to 5.5.2 errors if extensions are assumed but not supported.

To better understand this, it’s important to answer the following question:

Q: What is the ESMTP and how is it different from SMTP?

A: ESMTP (Extended SMTP) is an enhanced version of the original SMTP protocol that supports additional features like authentication, message size declaration, and encryption. 

🔖 Related Reading: SMTP vs. ESMTP: Key Differences and Why They Matter for Email Delivery

How to fix SMTP Error 555 5.5.2

Once you’ve pinpointed that SMTP Error 555 5.5.2 indicates a syntax error, the good news is—it’s often resolvable with just a couple of very careful tweaks. Here are the most common methods along with the details on how to fix it and have your emails accepted by the receiving server.

1. Validate email syntax before sending

One of the first things to check is if the email addresses you are using are unique and formatted correctly. This applies to both sender (MAIL FROM) and recipient (RCPT TO) fields.

• Use regex validation to ensure email strings match standard formatting rules
• Sanitize user inputs on all forms where emails are collected
• Prevent sending emails without full domain names (e.g., no john@)
• Disallow characters like commas, spaces, or special symbols not allowed in email addresses

2. Correct your SMTP command formatting

Even valid email addresses can cause problems if they’re not used correctly inside the SMTP command. Make sure you’re constructing your MAIL FROM and RCPT TO commands with the proper syntax and required angle brackets.

• Always use < > to enclose email addresses in SMTP commands
• Avoid sending bare addresses without brackets
• Manually inspect how your application constructs these commands
• Test your app’s SMTP conversation
  

Here’s an example of the correct format:

MAIL FROM:<[email protected]> RCPT TO:<[email protected]>

3. Check your email client or framework configuration

If you’re using a mail client (like Outlook, Thunderbird, or Apple Mail) or developing emails in a language like Python, PHP, or Node.js, syntax errors might originate from misconfigured libraries or headers.

• Review your mail library settings for fields like from_email, to_email
• Confirm your framework or plugin is wrapping email addresses correctly
• Make sure you’re not inserting raw email strings without sanitation
• Run a test email through a debugger or verbose mode

4. Review server logs and SMTP debug output

Sometimes, the best insights come from looking under the hood. Mail servers and email libraries often have logs or debug output that will show exactly what command failed—and why.

• Enable SMTP debugging or verbose output in your client or code
• Check your server logs for rejected commands
• Look for exact SMTP responses, not just generic “email failed” errors
• Identify if the failure occurs during MAIL FROM, RCPT TO, or DATA

Prevent SMTP syntax errors in the future with Warmy.io

Fixing SMTP errors is important—but preventing them is even better. Plus, it makes for long-term and sustainable email deliverability. With Warmy.io, you get more than just warm-up functionality—you gain access to a suite of features designed to keep your sending environment clean, validated, and error-free. 

Advanced seed lists comprised of genuine email addresses

Warmy.io’s seed lists provide a more human-like approach that sends strong positive engagement signals to different email providers. Since it also consists of real active email accounts, senders can be assured that these are formatted properly to avoid syntax errors and other inconvenient results.