Willi

Willi is a SMTP proxy. It transparently proxies SMTP sessions to an upstream SMTP server. The upstream server is selected based on the recipients (RCPT TO) of the incoming mail. Its main usage is as a proxy for MX servers.

Features

• Transparently proxy an SMTP session to another SMTP server. (Without storing the mail in a queue. If the upstream server rejects the message, the client will receive that reject immediately. No bounce message is sent.)
• Select upstream server based on mail recipient (RCPT TO).
• Read mapping from recipient to upstream server from MySQL database or CSV file.
• Map single recipients (foo@bar.com) or whole domains (bar.com).
• Flexible number and ordering of mappings.
• STARTTLS support in connection to clients.
• Use STARTTLS in connection to upstream server, if client used STARTTLS and upstream server supports it.
• Forward real client IP via XCLIENT, if upstream server supports it.

Installation

• Unpack willi-*.tar.gz,
• As root, run install.sh from the unpacked directory
• Willi is now installed, but will fail to start. It needs to be configured (see below).

Configuration

An example config file is provided in /opt/willi/etc/willi.conf.example. Copy it to /opt/willi/etc/willi.conf and change it according to the comments in the file itself.

Limitations

• If a client specifies multiple RCPT TO headers, only the first is used to select an upstream server. It will receive the complete SMTP session, including all other RCPT TO headers. If the upstream server does not accept mail for all recipients, it will reject the mail.
• Upstream servers must support SMTPUTF8, because Willi will always advertise it.
• If an upstream server does not support/allow XCLIENT from Willi, it only sees the proxy's IP. This can cause trouble with spam-filtering: If the upstream server blocks Willi's IP or greylists it, no client can send any mail to this server via Willi.
• If upstream server does not support STARTTLS, Willi falls back to plain connection (even if client sent STARTTLS).
• No support for authentication. This is by design, as Willi is primarily meant to be used for incoming mail.

Development

Work on the code

$> go get   # get dependencies
... develop ...
$> go build # build for local testing

Build a release

$> git tag -a v1.2.3 -m "v1.2.3"
$> make release # builds willi-v1.2.3.tar.gz
$> git push --tags

Optional: Install release

$> make install TARGET_HOST=my.server.com