We attempted a connection to your mail server on port 25 but your server did not respond. More than likely there is no email server at this address or you have a misconfigured firewall or spam filtering system.
If the SMTP Connect test is unable to connect to your mail server after 15 seconds we will report it as an error. For optimal email deliverability it is advisable that SMTP connections should not take longer than 15 seconds.
Here are the networks MxToolbox uses for SMTP Tests. Please add these to your firewall to avoid experiencing connection failures or false alerts.
22.214.171.124/28 (Netmask: 255.255.255.240)
126.96.36.199/27 (Netmask: 255.255.255.224)
188.8.131.52, 184.108.40.206, & 220.127.116.11
It is also possible your server is “Tar pitting.” Tar pitting is a technique used by some email servers to slow down spammers. The idea is that legitimate senders will wait longer to establish a connection than spammers will.
Here is a link to a link to the TechNet article with more information about Tarpitting
Our SMTP test cannot detect problems caused by Back Pressure, a system resource monitoring feature of the Microsoft Exchange Transport service. In order to be alerted for back pressure events you would need to add our end-to-end Mailflow Monitor for your server which can detect any type of email interruption and also provide historical performance data.
Here's a link to the TechNet article with more information about back pressure.
Are you managing your email delivery?
Email delivery is more than running an email server, or regularly checking for blacklists. To really ensure you customers get your email, you need to setup SPF, DKIM and DMARC. If you aren't manage your DMARC configuration and responses from email providers, you don't know if your customers are getting your email.
MxToolbox is YOUR expert on email deliverability. MxDelivery Center can help you setup and analyze your DMARC, DKIM and SPF to give you the insight you need to make email configuration changes and get your emails to your customer's.