Spam detection

1. A method of determining that a client is likely engaged in the sending of spam emails via a network node, the method comprising, at the network node:

defining a message size threshold and a message sending rate threshold, wherein defining the message size threshold comprises defining a message size between a minimum message envelope size and a maximum command size;

detecting opening of Simple Mail Transfer Protocol, SMTP, connections between a client device and an email server;

identifying messages sent from the client over the SMTP connections which exceed said message size threshold and counting the identified messages to determine a client email message sending rate, wherein the messages sent from the client over the SMTP connections comprise commands and one or more message envelopes; and

making an assumption that the client is engaged in the sending of spam emails if the client email message sending rate exceeds said message sending rate threshold.

2. A method according to claim 1 , wherein the network node is one of a VPN gateway, a router, a passive network monitoring system, a passive network monitoring sensor, and a firewall.

3. A method according to claim 1 , wherein the client is one of the client device, an application installed on the client device, or a user account associated with the client device or with the application.

4. A method according to claim 1 , comprising, in response to making the assumption that the client is engaged in sending spam emails, automatically blocking one or more SMTP ports to prevent the client from sending further email messages.

5. A method according to claim 1 , comprising, in response to making the assumption that the client is engaged in sending spam emails, sending a warning to the client.

6. A method according to claim 1 , wherein the message size threshold is 2 kB.

7. A method according to claim 1 , wherein the message size threshold is dynamically updated.

8. A method according to claim 1 , comprising measuring a size of RCPT TO commands or counting a number of RCPT TO commands sent from the client.

9. A method according to claim 1 , wherein one or more of said SMTP connections are Transport Layer Security, TLS, encrypted.

10. A system for determining that a client is likely engaged in the sending of spam emails via at least one network node device, the system comprising the at least one network node device configured to:

define a message size threshold and a message sending rate threshold, wherein the at least one network node device is configured to define the message size threshold between a minimum message envelope size and a maximum command size;

detect opening of Simple Mail Transfer Protocol, SMTP, connections between a client device and an email server;

identify messages sent from the client over the SMTP connections which exceed said message size threshold and count the identified messages to determine a client email message sending rate, wherein the messages sent from the client over the SMTP connections comprise commands and one or more message envelopes; and

make an assumption that the client is engaged in the sending of spam emails if the client email message sending rate exceeds said message sending rate threshold.

11. A system according to claim 10 , wherein the at least one network node device is one of a VPN gateway, a router, a passive network monitoring system, a passive network monitoring sensor, and a device implementing a firewall.

12. A method according to claim 10 , where the client is one of the client device, an application installed on the client device, or a user account associated with the client device or with the application.

13. A system according to claim 10 , wherein the at least one network node device is further configured to, in response to making the assumption that the client is engaged in the sending of spam emails, automatically block one or more SMTP ports to prevent the client from sending further email messages.

14. A system according to claim 10 , wherein the at least one network node device is further configured to, in response to making the assumption that the client is engaged in the sending of spam emails, send a warning to the client.

15. A system according to claim 10 , wherein the message size threshold is 2 kB.

16. A system according to claim 10 , wherein the at least one network node device is further configured to measure a size of RCPT TO commands or to count a number of RCPT TO commands sent from the client.

17. A system according to claim 10 , wherein, one or more of the SMTP connections are Transport Layer Security, TLS, encrypted.

18. A system according to claim 10 , wherein the at least one network node device comprises one or more processors configured to define the message size threshold and a rate of email messages sent from the client and to determine if the rate of email messages sent from the client exceeds the message sending rate threshold; or

wherein the at least one network node device comprises a counter configured to identify the messages that exceed the size threshold and to count a number of identified messages.

19. A system for determining that a client is likely engaged in the sending of spam emails via a system implemented by at least one network device, the system configured to:

define a message size threshold and a message sending rate threshold, wherein the system is configured to define the message size threshold between a minimum message envelope size and a maximum command size;

detect opening of Simple Mail Transfer Protocol, SMTP, connections between a client device and an email server;

identify messages sent from the client over the SMTP connections which exceed said message size threshold and count the identified messages to determine a client email message sending rate; and

make an assumption that the client is engaged in the sending of spam emails if the client email message sending rate exceeds said message sending rate threshold.

20. The system of claim 19 , wherein the at least one network device comprises a VPN gateway, a router, a passive network monitoring system, or a passive network monitoring sensor.

21. The system of claim 19 , wherein the system comprises a firewall implemented by the at least one network device.