Know these Gmail SMTP error codes to fix email delivery issues for your company’s domain

by DuoCircle


Not all your emails pass authentication checks and other filters, thus returning with error codes. These codes help diagnose and understand issues that occurred during email transmission, letting you fix them and resend the message. For instance, it could indicate if the problem is due to incorrect credentials, a blocked email, or a temporary server issue. Some error codes indicate security-related issues, such as failed authentication attempts or detection of spam or phishing attempts.

Here we are listing the most commonly encountered error codes in Gmail and ways to fix them.


How to read Gmail SMTP error codes?

An SMTP error code is a three-digit number. The first digit (1-5) shows the server’s response to the command, whether it accepted or encountered an error. The second digit indicates the type of issue, like a syntax or connection problem. The third digit provides specific details about the mail transfer status.


Common Gmail SMTP error codes


ERROR CODE- 550, “5.7.26”

This error code is returned when your email fails SPF and/or DKIM checks. The possible reasons for this failure are-

  • Multiple SPF records found for your sending domain.
  • Inclusion of wrong SPF and DKIM syntax.
  • The sender’s IP address is not listed in the SPF record.
  • Invalid or incomplete DKIM record.
  • Sending the same email to too many recipients at once. This is considered a spam-y behavior.


SMTP error codes


Here’s how you can resolve these issues

  • Using lookup tools to see configurational and syntactical issues.
  • Adding DKIM information to the sender’s email.
  • Switching to a reputed email server.


ERROR CODE- 550, “5.2.1”

Gmail returns this error code when a recipient’s mailbox receives a lot of emails at once. This error prompts when the recipient has many unread emails lying in their inbox.

Here’s how you can resolve this issue-

  • Recheck and ensure you have entered the correct email address in the ‘To;’ field.
  • Avoid sending emails to a lot of recipients at once.
  • Break large emails into multiple small emails.
  • Comply with the recipient’s email policy.


SMTP delivery


ERROR CODE- 550, “5.4.5”

This error code prompts when you exceed the daily sending limit. Gmail enforces daily sending limits to manage email traffic and prevent abuse. For SMTP delivery, the daily quota is set at 100 emails. When using the web interface, users can send up to 500 emails per day. For those using POP or IMAP clients, such as Microsoft Outlook, the maximum limit for sending emails at once is capped at 100.

Here’s how the issue can be fixed-

  • Send emails after 24 hours. If your account is temporarily disabled, it will be automatically enabled after 24 hours of reaching the daily limit. 
  • Contact your administrator or refer to the SMTP relay service error message if you wish to get your daily email allowance exceeded. 


SMTP Server

Image sourced from


Final words

Understanding SMTP error codes helps diagnose and resolve email delivery issues, highlighting the complexity of digital communication and the importance of email security. These codes, often considered technical jargon, are crucial in ensuring messages reach their destinations.

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