When you sync your inbox to our network we do the following actions:
Connect to your IMAP Port
Connect to your SMTP Port
Run your listed email address through many off the shelf email verifications tools (NeverBounce, MillionVerifier, etc...)
If any of these actions fails then we throw an error and will not allow you to connect to our network.
Typically when we throw a general "DNS Error" when syncing it typically means that the email verifications tools that we run in Step 3 mark your listed email address as either "Unknown" or "Invalid".
This error means that if we were to send an email to your email address we run the risk of the email bounce. We of course do not want this and err on the side of caution and will reject your connection.
Below are some helpful tools to utilize when troubleshooting your DNS. All you need to do is enter in your domain and each tool will highlight any records or settings that might be causing validation issues.
Google Admin Toolbox: Note only use this if you are utilizing a Google workspace or domain as it will throw false errors at you otherwise.
MX Toolbox: Overall this is the best tool to utilize for troubleshooting as it will clearly lay out anything that can be an issue.
There could be many reasons why your email is being marked as "Unknown" or "Invalid", but typically this is an issue with your DNS settings. Below are some ways to determine what might be the issue.
1. Verify That Your MX Records are Present:
MX Records are used to specify the mail server responsible for accepting email messages on behalf of a domain. In order for your email address to validate properly, you will need to ensure that all necessary records are present as validation tools will expect these records to exist in order to determine the validity of an address.
If you are using a Google workplace, these 5 settings need to be present with your DNS records. Depending on your domain provider, these will either be listed out as seen below or automatically configured behind the scenes. For a complete list of how to add/verify that these records are under your DNS, please refer to Googles Help Desk Page showing instructions for all domain providers.
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For the official documentation on how to verify/add your MX records to your DNS, please refer to Microsoft's Help Desk article on the subject.
If you are utilizing Microsoft 365 for your email workplace, you will need to verify that
your MX Records are being directed to the correct Outlook server. In order to do this.
Sign in to the Microsoft 365 admin center and select Show all > Settings > Domains.
In a new browser tab or window, sign in to your DNS hosting provider, and then find where you manage your DNS settings (e.g., Zone File Settings, Manage Domains, Domain Manager, DNS Manager).
Go to your provider's DNS Manager page, and add the MX record indicated in the admin center to your domain.
The Overview section under the domain section of the admin center will usually indicate to you that something is misconfigured with your DNS settings. The correct MX value to use can be found under the DNS records section as seen below.
For Custom Mail Servers:
Due to the various ways that a custom mail server can be configured, we cannot give specific instructions on how to resolve MX record issues.
2. Troubleshooting SMTP Transaction Timeout Errors
One of the ways that an email addresses can be verified as "Unknown" is due to a longer than expected response time when verification services are pinging a mail server.
The amount of time it takes to make an SMTP connection with a mail server is referred to as it's "Transaction time". If this transaction time takes longer than expected, verification services will not be able to determine for certain if there is a mail server associated with this domain and simply classify it as being an "Unknown" email address.
It typically takes under 7 seconds to successfully connect to a mail server over SMTP. If a connection takes longer than 7 seconds, this will indicate that either
The mail server in question is currently under heavy load, thus transaction times will take longer as other requests need to process.
The admin of this mailbox has some form of "Tarpitting" enabled under their mail server.
Tarpitting is a service on a computer system or server that purposely delays incoming connections as a form of spam mail protection. This feature is typically included under most mail server software but needs to be manually configured in order to be utilized.
If you are noticing that your mail servers response times are consistently taking longer than expected, you might have tarpitting enabled under your mail server and will need to adjust this setting in order to have email addresses under this domain validate properly.
If you are utilizing Google Workplace, longer than average SMTP connections are most likely due to heavy load on Googles end as there isn't any way to adjust these connection times. If you are having issues with "Unknown" email validations under a Google Workplace, we recommend writing into their customer service as there is little that we can do to troubleshoot this type of issue from our end.
Standard Microsoft 365 workplaces created under a custom domain will not have any form of tarpitting enabled by default. For workplaces configured with additional security protocols, tarpitting may be enabled if SMTP response times are longer than 5 seconds.
For a detailed explanation on how to adjust your SMTP response times, please refer to Microsoft's official documentation on the subject, but please note that this will not be enabled by default for a majority of Microsoft users
For Custom Mail Server Users:
A majority of mail server software will have options for adjusting your servers average SMTP response times.
If you are getting SMTP Transaction Time timeouts and you are using cPanel you may edit the Exim configuration at WHM to set Off the parameter for “Introduce a delay into the SMTP transaction for unknown hosts and messages detected as spam.” This will allow SMTP to connect as desirable.