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.
18.104.22.168/28 (Netmask: 255.255.255.240)
22.214.171.124/27 (Netmask: 255.255.255.224)
126.96.36.199, 188.8.131.52, & 184.108.40.206
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.