Types of SMTP Proxies
SMTP proxies come in a few fundamental flavors:
- Synchronous - each SMTP client connection causes the proxy to establish a single connection with a downstream mail server. See the open source smtp-gated project for an example of a synchronous SMTP proxy.
- Multiplexing - the proxy establishes downstream connections to the mail server only as needed, and by intelligently juggling a pool of SMTP connections; this juggling protects the downstream mail server from excessive connection concurrency.
- Transparent - the proxy is inserted into the network between clients and servers, masquerading itself in such a way that the client and server believe they are talking directly to each other, even though there is a proxy in the middle. The commercial "MailChannels Outbound" product from MailChannels implements a transparent proxy. Policy-based routing may be used to route SMTP traffic through a transparent SMTP proxy.
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