Electronic Mail

Transparent SMTP proxies

2009-06-26T00:57:00.001-07:00

When SMTP proxies are placed on the outgoing network, they typically are used to intercept all SMTP connections to make sure that unauthorized e-mail, spam, e-mail worms, etc. are not sent from the network. This is common on internet connections provided by hotels, company networks, and some ISPs and is discussed in RFC 5068.

More commonly is when SMTP proxies are placed on the incoming network where they typically are used in the integration of anti-spam techniques into MTAs, the creation of e-mail hubs for load balancing, or to forward e-mail from a company-wide domain name to individual department mail servers. By using a proxy instead of the more common store-and-forward method of relaying e-mail, backscatter can be greatly reduced.

An advantage of SMTP proxies is that they work with any already installed mail server. If one decides to switch to a different MTA, they can still use the same SMTP proxy.

Certain SMTP proxies implement connection management, which ensures that no matter how big a spike in traffic coming from the outside (e.g. a virus outbreak or a DoS attack), the back-end mail server will not receive more connections than it can handle.

Some proxies can implement throttling - where suspect SMTP connections are slowed down. When slowed down, spammers typically give up (since they need to deliver huge volumes of email), whereas legitimate senders don't mind waiting a few minutes. This is a lenient version of tarpitting.

Throttling is hard to implement in a non-SMTP proxy solution, since each throttling connection holds up a usually heavy-weight process or thread, whereas a proxy can be implemented as a very efficient asynchronous I/O solution which can juggle thousands of connections at once.

Another reason to use a proxy is that while the DNS allows multiple MX records which can be used to spread the load of incoming e-mail across multiple servers, DNS is not the best method for load balancing. Also, DNS can not easily allow more than about a dozen MX records, limiting the total number of incoming mail servers, which can be a problem for many large ISPs and corporations. By using transparent SMTP proxies, it is possible to improve the load balancing and increase the number of back-end mail servers.

SMTP proxy

2009-06-26T00:38:00.000-07:00

Transparent SMTP proxies are specialized Mail Transfer Agents (MTAs) that, similar other types of proxy servers, pass SMTP sessions through to other MTAs. They differ from regular MTAs in that they neither relay e-mail via store-and-forwarding nor delivering the e-mail to mailboxes। Instead, when they receive an SMTP session, they open up another SMTP session to a destination MTA. Any errors/status information from the destination MTA will be passed back to sending MTA through the proxy.

An advantage of SMTP proxies is that they work with any already installed mail server. If one decides to switch to a different MTA, they can still use the same SMTP proxy.

Electronic Mail

2008-02-08T01:30:00.000-08:00

E−mail Fundamentals
E−mail is probably the most attractive topic for users. Almost every user on the network uses
electronic mail (known by its acronym e−mail). Users do not know very much about UNIX, but they like to use, and they benefit from using, e−mail. Usually this is a love−hate relationship between users and e−mail; users love having an e−mail system, but they hate it when it does not work.
This relationship is exacerbated by the fact that only a few of them have any idea how e−mail actually works and really understand e−mail processes. The most common misunderstanding is to confuse e−mail with other popular network applications, like telnet or ftp.
And when the DNS issue arises during a discussion of e−mail, the confusion is complete.

It is not so easy to explain the principles of e−mail in a few words. For starters, e−mail can be
compared to regular ground mail service, or snail mail; this is the most helpful description that we have. If we use this analogy, though, we should also point out other services analogous to the
common network applications: telnet corresponds to the phone service, ftp to fax service, and DNS is an operator to help to resolve name/address/phone number relationships.

E−mail is based on several supporting programs and protocols that enable local and networkwide
e−mail service. The programs accomplish different tasks sequentially in the e−mail generation,
e−mail transfer (transportation), and e−mail delivery; the protocols define rules for the interprogram communications.

There are two categories of e−mail programs: "mail user agents," or MUA programs, and "mail
transport agents," or MTA programs. An MUA agent is any number of programs that users run to read, reply to, compose, and dispose of e−mail. These include, for example, the original UNIX mail program /bin/mail, the Berkeley mail or its System V equivalent mailx, and freely available
programs like mush, elm, pine, and mh, as well as other commercial programs.

An MTA agent is a program that handles mail delivery for many users and forwards e−mail between machines. The central mail transport program, the one most used today, is sendmail. On UNIX this is the default MTA program. Other mail programs are implemented around sendmail.
sendmail determines and invokes other delivery agents called Mailers to deliver e−mail and to further transfer e−mail. This is shown in Figure 20.1. Mailers are sometimes considered the third category of e−mail programs and named "delivery agents," or MDA programs. In this text mailers are treated as a part of the MTA program.