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 "Sending > 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 mainly appears when your email client fails to authenticate. This means no SMTP username or password was attempted. Often this can happen when a network filtering appliance or application (firewall, antivirus, etc) interferes with data transmission.
This error can also occur if an incorrect username or password combination is used when connecting to our servers. Please ensure you are using a combination from the "Sending > 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 you plan to use IP Authentication, please ensure you add the public IP address to the "Sending > IP Authentication" page.
If this error is encountered you should also ensure your sending domain is verified on the "Sending > 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 "Sending > 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 "Sending > 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 that IP address on this account is not allowed to send or This request is coming from an IP address that isn't in the IP allow list
This error means the "IP Allowlists" feature is enabled in the account and the IP address you are connecting from has not been added/authorized. To be able to send or make API calls, the IP will need to be added to the respective "IP Allowlist" feature located on the "Settings > Sending Options" section of the account.
550 Unable to verify sender address or 550 Sender verify failed or Could not complete sender verify
If you encounter one of these errors when trying to send an email, it indicates that the MX (incoming) mail server is either not responding or is non-existent.
Note: these errors do not refer to sender verification (Sending > Verified Senders)
The use of Bounce Notifications will determine if MX records are required and the scenarios include:
- If you have Bounce Notifications disabled, MX records for the sending domain are not required.
- If you have Bounce Notifications enabled and set it to deliver those to a designated email address, MX records are required for that domain.
- If you have Bounce Notifications enabled and set it to deliver those to the original sender, MX records are required for the sending domain.
We check that the domain has a functioning MX server when Bounce Notifications are enabled to ensure you can receive NDRs (bounce notifications) and delay notifications.
You can check the MX server by searching for the domain on MXToolbox.
Could not connect to SMTP host or 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 "Sending > SMTP Users" or "Sending > 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 is enabled and set so only the senders listed are allowed to send and the email address or domain that you are trying to send from has not been listed. You will need to add the address or domain to the "Settings > Sending Options > Restrict Senders" feature.
550 Sender address <address@example.com> is on your banned list
This error means the Restrict Senders feature is enabled and set to allow all senders except the ones listed and the email address or domain that you are trying to send from has not been listed. You will need to add the address or domain to the "Settings > Sending Options > Restrict Senders" feature.
550 Recipient <address@exmaple.com> is not on your allow list
This error means the Restrict Recipients feature is enabled and the email address you are attempting to send to has not been listed. You will need to add the address or domain to the "Settings > Sending Options > Restrict Recipients" 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 or 550 username rate limit exceeded
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 "Sending > SMTP Users" or "Sending > 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.
550 This ipaddr is blocked
This error is encountered if you are utilizing IP Authentication and the IP has been blocked. You can unblock the IP by going to the "Sending > IP Authentication" section, click on the IP address and then going to the "Tracking & Status" tab where you will see the "User Status" option.
421 Too many concurrent SMTP connections or Too many connections from that IP address
This error is encountered if you exceed the maximum number of connections from an IP address at a time. We allow up to 40 concurrent connections per IP address and you can send up to a maximum of 5,000 emails per connection.
550 "Administrative Prohibition" or "Relay Access Denied"
The 550 “administrative prohibition” or “relay access denied” bounce error is generated when a recipient mail server detects that our service hasn’t been given consent to send email on behalf of the sending domain, which is deemed suspicious and subsequently blocked.
This error can be resolved by adding your domain to the Verified Senders list. If the error message persists with a verified sending domain, have the recipient whitelist our sending IP addresses with their incoming mail server. Note: High volume accounts may have their recipients whitelist their dedicated IP address(es) instead of our entire range of sending IPs. Check the “Settings > Dedicated IPs” section of the account’s dashboard to identify the correct address(es) to whitelist.
If that fails, make sure your FROM address is the same as any domains listed in your Verified Senders.
550 Asked Not to Receive Your Email
The "550 Asked Not to Receive Your Emails" email rejection is generated when a recipient flags your message as spam via their inbox (for example, when they click the 'Report Spam' button in Yahoo). Once notified, SMTP2GO automatically blocks future outbound attempts to the recipient.
You can find these rejections (called 'rejects') by visiting the “Reports > Activity” section of your SMTP2GO account’s dashboard.
Unroutable Domain
When sending emails you may occasionally encounter the following type of message:
"550 unroutable domain: [domain]"
This error message essentially means that the domain name of the email address you tried to send an email to doesn't have a valid MX record. An MX record is the mailbox for a domain name, where emails for that domain name should be delivered.
So, if you tried to send an email to julie@baddomain123.com then either baddomain123.com doesn't have any MX record(s), or its MX record(s) are not reachable for some reason.
You can find out what the MX records are (if they exist) by searching on MXToolbox.
If no MX records exist, you should double-check to make sure you haven't made a spelling mistake.
If MX records do exist, you can try pinging them or running a quick telnet test (on port 25) to the MX record with the lowest-numbered priority.
Alternatively, get in touch with us, and we'll look into the situation.
You may need to contact your recipient by other means if you need to contact them quickly.
Domain of sender address does not exist
This may happen after adding CNAME records as per the Verified Senders setup.
The CNAME points to a subdomain on your DNS and, generally, this requires no extra action on your part.
However, if you are experiencing the above error, the solution is to add the subdomain used in the CNAME record (it begins with 'em') to your internal DNS, or to the particular software/filter that is generating this error.
It could be that your MX server (the server that hosts your incoming email) is rejecting the email as it doesn't know about the CNAME subdomain (the one beginning with 'em') that we advise you to add.
Support
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 and the IP address from your side that the connections will come to our inbound servers from.