The Proxies
The SMTP Proxy
SMTP protocol is lingua franca spoken by every mail server while exchanging emails with another server. Due to the complexity of the email system , this protocol is embellished with all the necessary provisions to cover every possible situation that may arise when email is handed from one system to another.
The email system is, by design, a rather slow business and servers may pop in and out of action. Consider the situation of a server A that has to deliver an email to server B. B is down for maintenance operations and cannot receive mail. In this case A has to queue the email and will attempt to deliver the email after a predetermined period of time. Another situation involves a server, C, that accepts email for a domain, let’s call it example.com. This domain, example.com has its own email server, D, but policy regulations forbid D to be directly connected to the internet – it will accept email only from C. C will accept email for example.com and will forward it to D. Having to deal with this kind of problems (we just described two examples) make the email system complex in contrast with the relative simplicity of POP3.
Our goal is to be able to filter emails in any circumstance, and that’s why Syneto’s SMTP proxy has to behave just like any ordinary mail server and implementing the same provisions to deal with situations like the ones described above. For the purpose of integrating it into a network architecture, the SMTP proxy should be considered a mail server (or MTA – Mail Transfer Agent) with the following peculiarities:
- it doesn’t receive mail for local recipients – as it doesn’t have any – and
- will forward all the email to its original destination, if it passes the email filters, antispam and antivirus systems
Like any mail server, it should be configured to accept email for a number of domains, should allow emails to pass through it to a general destination only if they originate from specific networks (this is called relaying), has a proper mail queue and should be perceived from outside like an ordinary email server.
The POP Proxy
The POP3 protocol is commonly used to retrieve emails from a server where the user has an account. Syneto’s POP3 proxy intercepts this communication and, when the email is fetched from the server, will collect and perform the necessary operations – scan it against malware, determine if it isn’t spam etc. If the email proves to be harmless it will be subsequently forwarded to user’s mail client; if the email is categorized as spam, contains a virus or fails other checks, the necessary measures are imposed upon it.
Syneto Proxies
Syneto devices proxy SMTP (tcp port 25) and POP3 (tcp port 110) traffic in a transparent manner. This means that the proxy intercepts the email traffic without any effort from the sender or receiver of the emails. Nevertheless, the appliance has to be in position to intercept this traffic, i.e. to be a gateway through which this traffic passes en route to destination.