chasquid supports email aliases, which is a mechanism to redirect mail from one account to others.
The aliases are configured per-domain, on a text file named aliases within
the domain directory. So like /etc/chasquid/domains/example.com/aliases.
The format is very similar to the one used by classic MTAs (sendmail, exim, postfix), but not identical.
Lines beginning with # are considered comments, and are ignored.
To create email aliases, where mail to a user are redirected to other
addresses, write lines of the form user: address, address, ....
The user should not have the domain specified, as it is implicit by the location of the file. The domain in target addresses is optional, and defaults to the user domain if not present.
For example:
# Redirect mail to pepe@ to jose@ on the same domain.
pepe: jose
# Redirect mail to flowers@ to the individual flowers.
flowers: rose@backgarden, lilly@pond
Destination addresses can be for a remote domain as well. In that case, the email will be forwarded using sender rewriting. While the content of the message will not be changed, the envelope sender will be the constructed from the alias user.
User names cannot contain spaces, ":" or commas, for parsing reasons. This is a tradeoff between flexibility and keeping the file format easy to edit for people. User names will be normalized internally to lower-case. UTF-8 is allowed and fully supported.
A pipe alias is of the form user: | command, and causes mail to be sent as
standard input to the given command.
The command can have space-separated arguments (no quotes or escaping expansion will be performed).
For example:
# Mail to user@ will be piped to this command for delivery.
user: | /usr/bin/email-handler --work
# Mail to null@ will be piped to "cat", effectively discarding the email.
null: | cat
If the aliased user is *, then mail sent to an unknown user will not be
rejected, but redirected to the indicated destination instead.
pepe: jose
*: pepe, rose@backgarden
If the same left-side address appears more than once, the last one will take precedence.
For example, in this case, the result is that pepe is aliased to jose, the
first line is effectively ignored.
pepe: juan
pepe: jose
When parsing aliases files, drop characters will be ignored. Suffix separators are kept as-is.
When doing lookups, drop characters will also be ignored. If the address has a suffix, the lookup will include it; if there is no match, it will try again without the suffix.
In practice, this means that if the aliases file contains:
juana.perez: juana
juana.perez+fruta: fruta
Then (assuming the default drop characters and suffix separators), these are the results:
juana.perez -> juana
juanaperez -> juana
ju.ana.pe.rez -> juana
juana.perez+abc -> juana
juanaperez+abc -> juana
juana.perez+fruta -> fruta
juanaperez+fruta -> fruta
This allows addresses with suffixes to have specific aliases, without having to worry about drop characters, which is the most common use case.
If different semantics are needed, they can be implemented using the hook.
Aliases files are read upon start-up and refreshed every 30 seconds, so changes to them don't require a daemon restart.
The resolver will perform lookups recursively, until it finds all the final recipients. There are recursion limits to avoid alias loops. If the limit (10 levels) is reached, the entire resolution will fail.
Commands are given 30s to run, after which it will be killed and the execution will fail. If the command exits with an error (non-0 exit code), the delivery will be considered failed.
The chasquid-util command-line tool can be used to check and resolve
aliases manually. It talks to the running server, so the response is fully
authoritative.
There is a hook that allows more sophisticated aliases resolution:
alias-resolve.
If it exists, it is invoked as part of the resolution process, and the results are merged with the file-based resolution results.
See the hooks documentation for more details.