Common Sending Errors

This article covers some common errors encountered when trying to send and how to resolve them.

535 Incorrect authentication data
The username and password combination is incorrect in your email client software. Please ensure you are using a combination from your account's Settings > SMTP Users page. We recommend you copy and paste the credentials from your SMTP2GO dashboard into your email setup to make sure they are an exact match.

Note: the combination you use to log in to your SMTP2GO account dashboard with is separate from the combination used to authenticate to our servers from the SMTP Users page.

550 Relay access denied - please authenticate
This error is mostly encountered if an incorrect username or password combination is used when connecting to our servers. Please ensure you are using a combination from the 'Settings > SMTP Users' section of your account. We recommend you copy and paste the credentials from there into your setup to make sure they are an exact match.

If this error is encountered you should also ensure your sending domain is verified on the Settings > Verified Senders > Sender Domains section of your account.

550 From header sender domain not verified
This error means you are trying to send from an email address at an unverified domain or an unverified single sender email. Verification is mandatory to authorize sending from a domain or from a specific address.

You need to verify your sending domain on the Settings > Verified Senders > Sender Domains section of your account to resolve this and that will also ensure emails pass SPF and DKIM improving deliverability.

If for some reason you cannot verify the sending domain, you will need to verify the single sender email address on the Settings > Verified Senders > Single sender emails section of your account. This will authorize sending but does not ensure your emails are SPF and DKIM signed by the domain. For further information, view our Verified Senders article or our Sender Domain vs Single Sender Emails article.

550 Verification failed for <email address>, 550 Unrouteable address, 550 Unroutable domain or 550 Unable to verify sender address
If you receive these errors then you will need to ensure that you are sending from a valid email address. Check that the address exists and the domain has working MX records. Further detail can be found in the specific articles for unable to verify sender address and unroutable domain.

Could not connect to SMTP host, Failed to connect to server or Connection refused 
If you encounter failed to connect or connection refused type errors, you will need to troubleshoot further in regard to why your side is unable to reach our servers - our Troubleshooting Connections Issues article covers recommended options in more detail with the common points including:

  • Connect via different a different port - your ISP may have the port blocked. The following ports are available:
    TLS or no encryption: available on ports 25, 2525, 8025, 587 and 80.
    SSL: available on ports 465, 8465 and 443.
  • Ensure there is no firewall or antivirus software blocking your connections and check the port you're using is open.
  • As a troubleshooting idea, try to temporarily connect to one of our static IPs instead of 'mail.smtp2go.com'.

550 That smtp username is not allowed to send or 550 That IP address is not allowed to send
These errors are encountered if the account is closed (check to make sure the account is active) or if the SMTP User or IP has the status of blocked. You can check the status in your Settings > SMTP Users or Settings > IP Authentication section, select the username or IP, choose the Tracking & Status tab then view "User Status".

550 That SMTP sender is not on your allowed list
This error means the Restrict Senders feature has been enabled and the email address or domain that you are trying to send from has not been listed/allowed. You will need to add the address/domain to the Settings > Sending Options > Restrict Senders feature. 

552 Your monthly email allowance is exhausted
Your monthly quota has been used. You will need to upgrade to a higher volume plan for more emails per month to continue sending or you will need to wait until your monthly quota renews. View our subscription differences article for more information.

550 Your username has exceeded the allowed sending rate or 550 Your IP address exceeded the allowed sending
This error means you have reached the set rate limit for the SMTP User or IP. This limit can be updated for the SMTP User or IP on the Settings > SMTP Users or IP Authentication page > select the username or IP and adjust the rate limit. Please note that this is a rate (or speed) limit and not a quota limit.


If you continue to encounter an error when trying to send then please contact our award-winning support team for further assistance. Please provide as much information as possible including your current outgoing SMTP settings, the error you get and the software or device you are trying to set up.

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