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Introduction to exim

Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the @@ -31,206 +31,206 @@

Basic Features

- Many configuration options can be given as expansion strings, - and as these can include file lookups, much of Exim's - operation can be made table-driven if desired. For example, - it is possible to do local delivery on a machine on which the - users do not have accounts. + Many configuration options can be given as expansion strings, + and as these can include file lookups, much of Exim's + operation can be made table-driven if desired. For example, + it is possible to do local delivery on a machine on which the + users do not have accounts.
- Regular expressions are available in a number of configuration - parameters. + Regular expressions are available in a number of configuration + parameters.
- Domain lists can include file lookups, making it possible to - support a large number of local domains. + Domain lists can include file lookups, making it possible to + support a large number of local domains.
- The maximum size of message can be specified. + The maximum size of message can be specified.
- Exim can handle a number of independent local domains on the - same machine; each domain can have its own alias files, - etc. These are commonly called virtual domains. + Exim can handle a number of independent local domains on the + same machine; each domain can have its own alias files, + etc. These are commonly called virtual domains.
- Exim contains an optional built-in mail filtering - facility. This enables users to set up their own mail - filtering in a straightforward manner without the need to run - an external program. There can also be a system filter file - that applies to all messages. + Exim contains an optional built-in mail filtering + facility. This enables users to set up their own mail + filtering in a straightforward manner without the need to run + an external program. There can also be a system filter file + that applies to all messages.
- Periodic warnings are automatically sent to messages' senders - when delivery is delayed -- the time between warnings is - configurable. + Periodic warnings are automatically sent to messages' senders + when delivery is delayed -- the time between warnings is + configurable.
- A queue run can be manually started to deliver just a - particular portion of the queue, or those messages with a - recipient whose address contains a given string. + A queue run can be manually started to deliver just a + particular portion of the queue, or those messages with a + recipient whose address contains a given string.
- Exim can be configured to run as root all the time, except - when performing local deliveries, which it always does in a - separate process under an appropriate uid and - gid. Alternatively, it can be configured to run as root only - when needed; in particular, it need not run as root when - receiving incoming messages or when sending out messages over - SMTP. + Exim can be configured to run as root all the time, except + when performing local deliveries, which it always does in a + separate process under an appropriate uid and + gid. Alternatively, it can be configured to run as root only + when needed; in particular, it need not run as root when + receiving incoming messages or when sending out messages over + SMTP.

Incoming SMTP

- SMTP calls from specific machines, optionally from specific - idents, can be locked out, and incoming SMTP messages from - specific senders can also be locked out. + SMTP calls from specific machines, optionally from specific + idents, can be locked out, and incoming SMTP messages from + specific senders can also be locked out.
- Messages on the queue can be `frozen' and `thawed' by the - administrator. + Messages on the queue can be `frozen' and `thawed' by the + administrator.

Outgoing SMTP

- Exim can perform multiple deliveries down the same SMTP - channel after deliveries to a host have been delayed. + Exim can perform multiple deliveries down the same SMTP + channel after deliveries to a host have been delayed.
- Exim can be configured to do local deliveries immediately but - to leave remote deliveries until the message is picked up by a - queue-runner process. This increases the likelihood of - multiple messages being sent down a single SMTP connection. + Exim can be configured to do local deliveries immediately but + to leave remote deliveries until the message is picked up by a + queue-runner process. This increases the likelihood of + multiple messages being sent down a single SMTP connection.
- When copies of a message have to be delivered to more than one - remote host, up to a configured maximum number of remote - deliveries can be done in parallel. + When copies of a message have to be delivered to more than one + remote host, up to a configured maximum number of remote + deliveries can be done in parallel.

Local Deliveries

- Exim stats a user's home directory before looking for a - `.forward' file, in order to detect the case of a - missing NFS mount. + Exim stats a user's home directory before looking for a + `.forward' file, in order to detect the case of a + missing NFS mount.
- There is support for multiple user mailboxes controlled by - prefixes or suffixes on the user name, either via the - filter mechanism or through multiple - `.forward' files. + There is support for multiple user mailboxes controlled by + prefixes or suffixes on the user name, either via the + filter mechanism or through multiple + `.forward' files.

Monitoring and Performance Tools

- The Exim Monitor is an optional extra; it displays information - about Exim's processing in an X window, and an administrator - can perform a number of control actions from the window - interface. + The Exim Monitor is an optional extra; it displays information + about Exim's processing in an X window, and an administrator + can perform a number of control actions from the window + interface.
- There are a set of tools for summarising the queue, determining - what each exim process is currently doing, examining the - deliveries hints databases, and summarising the log files into - a concise report on activity. + There are a set of tools for summarising the queue, determining + what each exim process is currently doing, examining the + deliveries hints databases, and summarising the log files into + a concise report on activity.

SPAM/UCE/UBE Limitation Features

- Senders can be blocked using a variety of methods, including - the ability to apply a set of username or regular expression - patterns to incoming mail from particular (or all) domains. + Senders can be blocked using a variety of methods, including + the ability to apply a set of username or regular expression + patterns to incoming mail from particular (or all) domains.
- All addresses can be checked for validity during the SMTP - transaction thus allowing the elimination of mail from - non-existent domains. + All addresses can be checked for validity during the SMTP + transaction thus allowing the elimination of mail from + non-existent domains.
- Relaying can be tightly controlled based on sending host, - network or sending/recipient domains. + Relaying can be tightly controlled based on sending host, + network or sending/recipient domains.
- If all else fails the system filter can be used to deal with - messages based on arbitrary conditions that you can program in - (for example block mail with a particular X-Mailer: - header. + If all else fails the system filter can be used to deal with + messages based on arbitrary conditions that you can program in + (for example block mail with a particular X-Mailer: + header.

Limitations

- Exim is written in ANSI C. This should not be much of a - limitation these days. However, to help with systems that - lack a true ANSI C library, Exim avoids making any use of the - value returned by the `sprintf()' function, which is - one of the main incompatibilities. It has its own version of - `strerror()' for use with SunOS4 and any other - system that lacks this function, and a macro can be defined to - turn `memmove()' into `bcopy()' if - necessary. + Exim is written in ANSI C. This should not be much of a + limitation these days. However, to help with systems that + lack a true ANSI C library, Exim avoids making any use of the + value returned by the `sprintf()' function, which is + one of the main incompatibilities. It has its own version of + `strerror()' for use with SunOS4 and any other + system that lacks this function, and a macro can be defined to + turn `memmove()' into `bcopy()' if + necessary.
- Exim uses file names that are longer than 14 characters. -
- Exim is intended for use as an Internet mailer, and therefore - handles addresses in RFC 822 domain format only. It cannot - handle bang paths, though simple two-component bang - paths can be converted by a straightforward rewriting - configuration. + Exim is intended for use as an Internet mailer, and therefore + handles addresses in RFC 822 domain format only. It cannot + handle bang paths, though simple two-component bang + paths can be converted by a straightforward rewriting + configuration.
- Exim insists that every address it handles has a domain - attached. For incoming local messages, domainless addresses - are automatically qualified with a configured domain - value. Configuration options specify from which remote systems - unqualified addresses are acceptable. + Exim insists that every address it handles has a domain + attached. For incoming local messages, domainless addresses + are automatically qualified with a configured domain + value. Configuration options specify from which remote systems + unqualified addresses are acceptable.
- The only external transport currently implemented is an SMTP - transport over a TCP/IP network (using sockets), suitable for - machines on the Internet. However, a pipe transport is - available, and there are facilities for writing messages to - files in `batched SMTP' format; this can be used to send - messages to some other transport mechanism. Batched SMTP input - is also catered for. + The only external transport currently implemented is an SMTP + transport over a TCP/IP network (using sockets), suitable for + machines on the Internet. However, a pipe transport is + available, and there are facilities for writing messages to + files in `batched SMTP' format; this can be used to send + messages to some other transport mechanism. Batched SMTP input + is also catered for.

Nigel Metheringham

Introduction to exim

Introduction to exim

Basic Features

Incoming SMTP

Outgoing SMTP

Local Deliveries

Monitoring and Performance Tools

SPAM/UCE/UBE Limitation Features

Limitations

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