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Ever wondered what your email delivery errors mean?

These errors can be extremely helpful in figuring out why your emails have bounced or have been rejected. We’re here to clear up some of the meanings behind the most frequently occurring delivery errors so that you can improve your sending in the future.

Hard / soft bounces

To start off, it’s important to know the difference between a hard bounced email, and a soft bounced email. A hard bounce is a permanent error and a soft bounce is a temporary one. An example of a hard bounce is an email sent to an invalid email address. A soft bounce may be caused by a temporary issue with the recipient server, for example if their DNS server is down, or their mailbox may be full.

Example of a hard bounce: The email account that you tried to reach does not exist

Example of a soft bounce: Recipient address rejected: Mailbox full

A bounce is a final result for an email, so regardless of whether it’s a soft or a hard bounce, our servers will not continue attempting to send the email. Once an email has been hard bounced, it is also called a “rejected email”. If you find a hard bounce that you think may work again, you can unblock it from the ‘Reports > Suppressions‘ page in your SMTP2GO control panel.

Here are some of the most common delivery errors that we see, and how you can resolve them:

Connection Timed OutFirst of all, check that your firewall isn’t restricting your emails from being sent. Then, try testing our other port numbers. We recommend using port number 8025, 587, 80 or 25 with TLS. SSL is available on ports 465, 8465 and 443.

Too Many Connections- When sending emails through your SMTP2GO account, you can have up to 20 concurrent SMTP connections. You’ll need to check that your concurrent connections setting isn’t set to any higher than this. You can also try using an SMTP Pipelining Protocol. If this is something that you’re interested in, you may want to check out how to use Postfix for email sending.

450 DNS Lookup Fail- This error is often generated when the domain name does not have a valid MX Record. You can use MX Toolbox to check this. Be sure to use the “DNS Propagation” option to see if it’s a problem with all of the DNS servers. If there is an issue here, you’ll need to speak to your DNS provider who can fix this for you.

552 5.2.0 u2k81t0253BLhLf012k9TP Suspected spam message rejected. IB703- This error is sometimes generated by emails sent to the Bigpond (Australia) servers “@bigpond.com” when two or more recipients are included in the TO header. To resolve this, simply include one recipient email address in the TO header, and CC or BCC and other recipients.

This email matches a profile the Internet community may consider spam. Please revise your message before resending.- It may be that something in the body of your email triggered a spam filtering software to reject your message. We recommend testing the content of your email using Mail-Tester.  Body content that triggers spam filters may include too many capitalised words, poor grammar and or spelling, overuse of punctuation, lack of an unsubscribe link or incorrect HTML links.

No immediate delivery: load average- Our servers are overworked, but we have queued your email and it will be sent soon.

Previously hard-bounced- This means that an email to this recipient email address previously bounced. Our servers automatically block hard-bounced email addresses from being sent to. To unblock this email address, just to go the “Suppressions” page of your SMTP2GO dashboard.

521 5.2.1 : AOL will not accept delivery of this message.- It is possible that your email is too spam-like or that your IP address is on a blacklist. Try testing your email content using Mail-Tester.

550 You are not allowed to send from that email address> #SMTP#- Check the “Restricted Senders” list on your SMTP2GO dashboard to ensure that there are no restrictions on the email addresses that you are allowed to send from. If this looks ok, check the SPF record for the sending domain to make sure that include:spf.smtp2go.com is included in the record.

550 5.7.750 Service unavailable / Client blocked from sending from unregistered domains- Check to see if your IP address or domain is blacklisted.

554 message/partial not supported/ 554 5.6.1 Messages of type message/partial are not supported- If you are sending from a printer or a scanner, it may be that the content exceeds the default scan size of 2MB. First of all, turn of or disable the “multipart” option on the copier or scanner. If that doesn’t work, increase the default scan size to around 10MB.

x@xyz.com has asked to not receive your email- This means that the recipient has marked your email as spam. Once an email address has marked a sender as spam, our servers automatically block that address from being sent to anymore. If you believe that this was done in error, you can ask the recipient to remove your email from their spam folder, and then you can unblock their email address from the “Suppressions” tab on your SMTP2GO dashboard.

503 unable to verify sender address- Please check your sending email address to ensure that it exists and is spelled correctly. A sender email address must have a functioning inbox so that NDRs can be delivered. If you have not yet created this email address, please setup a mailbox. If the email address does exist, it may be that there is an issue with the MX record (this can be temporary). You’ll need to speak to your DNS provider to ask them to resolve this.

#< #5.0.0> #SMTP#- This is normally an issue with the sender domain’s SPF record. Check the SPF record for the sending domain to make sure that include:spf.smtp2go.com is included in the record.

Put into go slow queue- If you see this error, it means that the recipient server has requested that we delay or slow down sending. The email will be sent within the next 30 minutes. In the meantime, please check your SPF and DKIM records to ensure that they are set up correctly as this can often trigger the issue.

550 this address: <x@x> is blocked by manual selection. To clear this you must use the api… or contact support- If you see this error, you’ll need to use the blocklist API to resolve it. Alternatively, please get in touch and we can unblock it for you.

550 Maximum line length exceeded (see RFC 5322 2.1.1).- Please update your DKIM record. You can also turn on the unsubscribe footer (which can be left blank) which will force the entire body text to be compliant with RFC 5322 2.1.1.

We’ve found lots of messages from smtphost.com- If you see this error it may suggest an IP reputation issue, please get in touch and we can resolve it for you. 

550 relay access denied – please authenticate- Please ensure that your SPF record and DKIM record are up to date and that the SPF record includes the entry include:spf.smtp2go.com

These are just some of the most common delivery errors that you may encounter when sending emails. If you have any doubts or questions regarding an error that you have received, please feel free to get in touch and we can advise you on the best way to resolve it.

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