Failmail is an SMTP proxy server that deduplicates and summarizes the emails it receives. It typically sits between an application and the SMTP server that application would use to send alerts or exception emails, and prevents the application from overwhelming its operators with a flood of email.

Its primary goals are:

1. to help the human operators of an application or system under observation to understand the errors and notifications that come from the system
2. to prevent an upstream SMTP server from throttling or dropping messages under a high volume of errors/alerts
3. to be easier to set up and use than its predecessor, failnozzle, which had strict assumptions about the nature and format of incoming messages

Failmail is language/framework/tool-agnostic, and usually only requires a small configuration change in your application.

To download and build from source:

$ export GOPATH=...
$ go get -u github.com/mpapi/failmail
$ $GOPATH/bin/failmail

Or, for 64-bit Linux, you can grab a binary of the latest release.

See failmail --help for a full listing of supported options.

By default, failmail listens on the local port 2525 (--bind="localhost:2525"), and relays mail to another SMTP server (e.g. Postfix) running on the local default SMTP port (--relay="localhost:25"). It receives messages and rolls them into summaries based on their subjects, sending a summary email out 30 seconds (--wait=30s) after it stops receiving messages with those subjects, delaying no more than a total of 5 minutes (--max-wait=5m). Each summary is sent to the union of all of the recipients of the messages in the summary.

Any summary emails that it can't send via the server on port 25, it writes to a maildir (--fail-dir="failed"; readable by e.g. mutt, or any text editor). If the --all-dir option is given, failmail will write any email it gets to a maildir for inspection, debugging, or archival.

The value of a failmail setting is determined as follows:

1. If a config flag is given on the command line for that option, the value of that flag is used. Flags may be given with one hyphen or two (-flag or --flag) and with an equals sign before the value or not (-flag=value or -flag value).
2. Otherwise, if a config file is given (via the --config flag on the command line), and the setting is present in the file, its value from the file is used.
3. Finally, the failmail default is used. (The values of these defaults are shown when running failmail --help.)

In general, the command-line flag spells the setting name with hyphens, and the config file uses underscores. For instance, --max-wait=5m is specified in a config file as max_wait = 5m.

The settings are described below:

• --all-dir (default: none)

  write all sends to this maildir

• --batch-expr (default: "{{.Header.Get \"X-Failmail-Split\"}}")

  an expression used to determine how messages are batched into summary emails

  (See "Configuring message batching" below.)

• --bind-addr (default: "localhost:2525")

  local bind address

• --bind-http (default: "localhost:8025")

  local bind address for the HTTP server

• --config (default: none)

  path to a config file

• --credentials (default: none)

  username:password for authenticating to failmail

• --fail-dir (default: "failed")

  write failed sends to this maildir

• --from (default: "failmail@$(hostname)")

  from address

• --group-expr (default: "{{.Header.Get \"Subject\"}}")

  an expression used to determine how messages are grouped within summary emails

  (See "Configuring message batching" below.)

• --max-wait (default: 5m0s)

  wait at most this long from first message to send summary

• --pidfile (default: none)

  write a pidfile to this path

• --relay-addr (default: "localhost:25")

  relay server address

• --relay-all

  relay all messages to the upstream server

• --relay-password (default: none)

  password for auth to relay server

• --relay-user (default: none)

  username for auth to relay server

• --shutdown-timeout (default: 5s)

  wait this long for open connections to finish when shutting down or reloading

• --socket-fd (default: 0)

  file descriptor of socket to listen on

• --tls-cert (default: none)

  PEM certificate file for TLS

• --tls-key (default: none)

  PEM key file for TLS

• --version

  show the version number and exit

• --wait-period (default: 30s)

  wait this long for more batchable messages

• --write-config (default: none)

  path to output a config file

The --batch-expr and --group-expr options are template strings used to determine the way that messages are batched into summary emails and grouped within those summary emails. They are Go templates evaluated in the context of a Go mail.Message. Two messages whose expressions evaluate to the same string are treated as belonging to the same group or batch.

Two functions are available in addition to the usual template functions:

• match, which takes a regular expression and a string and returns the leftmost match of the pattern, e.g:

      {{match "\[\w+\]" (.Header.Get "Subject")}}
  

  will batch messages like "[bos] error in db" and "[bos] error in web" together, but "[nyc] error in db" separately.

• replace, which takes a regular expression, a string, and a replacement, and returns the string with matches of the regular expression replaced by the replacement string, e.g.:

      {{replace "\[\w+\]" (.Header.Get "Subject") "*"}}
  

  will batch messages like "[bos] error in db" and "[nyc] error in db" together (treating them both as "[***] error in db"), but "[bos] error in web" separately.

See the examples directory for code snippets for your favorite programming language/logging framework. (If there's one that you'd like to see, feel free to open an issue or submit a pull request.)

For now, the best way to run failmail in production is via a tool like supervisord, runit, or daemontools. Here's an example supervisord configuration:

[program:failmail]
command=/usr/local/bin/failmail --relay="smtp.mycompany.example.com:25"
autorestart=true
autostart=true
stderr_logfile=/var/log/failmail.err
stdout_logfile=/var/log/failmail.out

$ export GOPATH=...
$ go get -d github.com/mpapi/failmail
$ cd $GOPATH/src/github.com/mpapi/failmail
$ ...
$ go fmt
$ go build
$ go test
$ git commit && git push ...

Other helpful testing tools include the smtp-source and smtp-sink commands, which are part of Postfix (and are in the Debian postfix package):

$ smtp-source -v -m 50 127.0.0.1:2525  # send 50 messages to failmail

The Python debugging SMTP server is also useful as an upstream server:

python -m smtpd -n -c DebuggingServer localhost:3025