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Even if it does, it might not be the host that is supposed to receive the mail. The distinction between header and envelope addresses is important because mail routers may process them differently. An example will help explain the difference between the two. Suppose you have a paper memo that you want to send to your colleagues Mary and Bill at the Gonzo corporation, and Ted and Ben at the Whizzer company. You give a copy of the memo to your trusty mail clerk Alphonse, who notes the multiple recipients.
Since he's a clever fellow who wants to save your company 29 cents, he makes two copies of the memo and puts each in an envelope addressed to the respective companies rather than sending a copy to each recipient. On the cover of the Gonzo envelope he writes Mary and Bill, and on the cover of the Whizzer envelope he writes Ted and Ben. When his counterparts at Gonzo and Whizzer receive the envelopes, they make copies of the memo and send them to Mary, Bill, Ted and Ben, without inspecting the addresses in the memo itself. As far as the Gonzo and Whizzer mail clerks are concerned, the memo itself might be addressed to the pope; they only care about the envelope addresses.
SMTP clients and servers work in much the same way. Suppose that joe gonzo. The recipient list in the letter's headers may look like this:.
The SMTP client at gonzo. When it's ready to list the recipients the envelope address , what should it say? If it gives both recipients as they are listed in the To: line above the header address , Betty will get two copies of the letter because the whizzer. The same problem occurs if the gonzo. The zippy.www.cantinesanpancrazio.it/components/pyfuwyn/179-come-mettere.php
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The solution is the same one that Alphonse and his fellow mail clerks used. The gonzo. The complete recipient list is still in the letter's headers, but those are inside the envelope, and the SMTP servers at gonzo. In this example, the envelope for the whizzer. Aliases illustrate another reason why header and envelope addresses differ. Suppose you send mail to the alias homeboys, which includes the names alphonse, joe, betty, and george.
In your letter you write To: homeboys. However, sendmail expands the alias and constructs an envelope that includes all of the recipients. Depending on whether those names are also aliases, perhaps on other hosts, the original message might be put into as many as four different envelopes and delivered to four different hosts.
In each case the envelope will contain only the name of the recipients, but the original message will contain the alias homeboys expanded to homeboys your. A final example shows another way in which envelope addresses may differ from header addresses. Suppose you have a file named letter that looks like this:.
You will receive the letter even though your login name doesn't appear in the letter's headers because your address was on the envelope. Unless told otherwise with the -t flag , sendmail constructs envelope addresses from the recipients you specify on the command line, and there isn't necessarily a correspondence between the header addresses and the envelope addresses. To better understand how to set up sendmail, you need to know what different jobs it does and how those jobs fit into the scheme of MUAs, MTAs, mail routers, final delivery agents, and SMTP clients and servers.
However, it does not do final delivery of mail. How does sendmail do this? This somewhat cryptic file tells sendmail exactly how you want various kinds of mail handled. It is extremely flexible and powerful, and at first glance seemingly inscrutable. However, one of the strengths of V8 sendmail is its set of modular configuration file building blocks. Most sites can easily construct their configuration files from these modules, and many examples are included. Writing a configuration file from scratch is a daunting task and you should avoid it if you can.
Because SMTP is a connection-oriented protocol, there is always a client and a server also known as a sender and a receiver.
When your system boots and it starts in daemon mode, it runs continuously, listening on the SMTP port for incoming mail. One thing sendmail doesn't do is final delivery. The additional complexity of final mail delivery is the last thing sendmail needs. The most important are the aliases file and the configuration file, sendmail.
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The statistics file, sendmail. That's all that needs to be said about sendmail. For instance, each Internet site is required to have a valid address postmaster to which mail problems may be reported.
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Most sites don't have an actual account of that name but divert the postmaster's mail to the person or persons responsible for e-mail administration. For instance, at the mythical site gonzo. This line tells sendmail that mail to postmaster should instead be delivered to the login names joe and betty. In fact, those names could also be aliases:.
In all of these examples, the alias name is the part on the left side of the colon, and the aliases for those names are on the right side. In the previous example, to resolve the alias postmaster, sendmail first expands it into the list of recipients firstshiftops, secondshiftops, and thirdshiftops and then expands each of these into the final list, joe, betty, lou, emma, ben, mark, and clara.
Although the right side of an alias may refer to a remote host, the left side may not. The alias joe: joe whizzer. Aliases may be used to create mailing lists in the example above, the alias postmaster is in effect a mailing list for the local postmasters. For big or frequently changing lists, you can use the :include: alias form to direct sendmail to read the list members from a file.
If the aliases file contains the line:. This is handy for mailing lists that change frequently, or those managed by users other than the postmaster. If you find a user is asking for frequent changes to a mail alias, you may want to put it under her control. The aliases file also may be used to send the contents of e-mail to a program. For instance, many mailing lists are set up so that you can get information about the list or subscribe to it by sending a letter to a special address, list -request.
The letter usually contains a single word in its body, such as help or subscribe, which causes a program to mail an information file to the sender. Suppose that the gonzo mailing list has such an address called gonzo-request:.
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You can also create an alias that causes sendmail to send mail to files. Aliases that specify files cause sendmail to append its message to the named file. The easiest way to show you how to set up sendmail is to use a concrete example.
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However, because sendmail runs under many different versions of UNIX, your system may vary from the examples shown below. For the sake of concreteness, these examples assume that you're setting up sendmail on a Solaris 2. First you must obtain the source and compile sendmail. Next you must choose a sendmail. Then you must test sendmail and its configuration file. Finally, you must install sendmail, sendmail.
You may also have to make changes to your system's DNS database, but that won't be covered here. This is the most recent version available as this book goes to press, and it is the version documented in the O'Reilly book sendmail. However, if your site is on the Internet and you want to obtain the absolutely latest version, ftp to the host ftp.
Use the following steps to download it:. Note that the exact name of the files to download differs depending on the current version of V8 sendmail, in this case version 8. Also, because the files are compressed, you must give ftp the binary command before transferring them. Note too that you should include your complete e-mail address as the password, for instance, mylogin gonzo. You may also wish to download the extended documentation and the support programs, which in this example would have been contained in the files sendmail. Z and sendmail. Now that you've got the source, you need to unpack it.
Because it's a compressed tar image, you must first decompress it and then extract the individual files from the tar archive. Now you're almost ready to compile sendmail, but first read the following files, which contain the latest news pertinent to the specific release of sendmail you've downloaded:.
As you can see, because sendmail runs on a variety of hosts and operating systems, a Makefile is provided for many UNIX variants. Since in this example we're assuming a Sun Microsystems Solaris system, we'll use Makefile. Solaris to compile sendmail. But before we type make, we should look at the files conf.
You probably won't want to change much in conf. Solaris is a different story. At the very least you should make sure that the correct version of the Solaris operating system is defined.
In this case, since we're compiling for Solaris 2.