Blocked Internet Ports List

Cumberland Connect promotes a fair and open-access Internet experience. However, to protect our subscribers and be good stewards of the greater Internet community, we do block certain Internet ports that are widely known to be involved in abuse, SPAM, or other malicious activities. Below is information about the ports we are currently blocking. Additional ports may be blocked in the future as new information becomes available.

Port Transport Protocol Direction Reason for Block
0 TCP/UDP Reserved Outbound Port is reserved and should not be used by any network applications.
25 TCP SMTP Outbound Port is not secure and allows unencrypted, unauthenticated email service.

Botnet spammers can use open subscriber devices to relay SPAM emails.

See the note below for more information.


Port 25/TCP – Simple Mail Transport Protocol (SMTP) uses port 25 as the default port to send email. TCP port 25 is blocked on dynamically assigned IP addresses. SMTP on port 25 is unencrypted and unauthenticated and is a target for the bad guys to use systems as mail relay for SPAM.

Cumberland Connect recommends that all end user devices use Port 587/TCP for SMTP. Port 587 sends emails that are both encrypted and authenticated.

Wanting to host your own email server? We recommend subscribing to a free or paid (depending on volume) SMTP service available on the Internet. Search for “free smtp services” on your favorite search engine to get started.

If you would like to learn more about the differences between SMTP port 25 and 587, CBTNuggets has an informative article to get started.