I love Debian, and Exim4 seems to ‘just work’ for me most of the time, so I tend to use it for my MTA by preference.  Debconf handles the basic options for Exim4 pretty well, and usually I don’t need to mess with anything.

However, on one of my VPS’s I wanted to do what I used to refer to as masquerading.  I use the term to refer to having an SMTP masquerade as a different host on outbound e-mail addresses automatically.  So the server may be fred.example.net, but all outgoing mail comes from user@example.net.  It’s common if you want to handle the return mail via some other route – and for me I do.  My servers are in the darkstorm.co.uk domain, but I don’t want them handling mail for somehostname.darkstorm.co.uk, and I don’t want to have to configure every user with a different address, I just wanted a simple way to get Exim to masquerade.  Additionally, I only want external outbound mail re-writing, mail which is staying on the server should remain untouched.  This allows bob to mail fred on the server, and fred to reply without the mail suddenly going off the server, but if bob mails bill@example.org, then his address is re-written correctly.

I think I spent some time looking at this a couple of years ago, and had the same experience as recently – it’s a bit frustrating tracking down the best place to do it.  Firstly, Exim doesn’t have a masquerade option as such, and the manual doesn’t refer to masquerading in that way.  What it does have is an extensive rewriting section in the config file and support for doing that rewriting in various ways.

On top of this, the Debian configuration of Exim can be a little daunting at first, and how you achieve the configuration may depend on whether you’re using a split config or the combined config.

Anyway, enough rambling, you can get Exim to rewrite outgoing mail / masquerade by setting one macro.  This works on Debian 6 (Squeeze) with Exim4, but I assume it’ll work with Exim4 on any Debian installation.

Create (or edit)

/etc/exim4/exim4.conf.localmacros

Add the following line,

REMOTE_SMTP_HEADERS_REWRITE = *@hostname.example.net ${1}@example.net

Rebuild the Exim config (might not be essential but I do it every time anyway),

update-exim4.conf

and then recycle Exim (reload might work, but I tend to recycle stuff),

/etc/init.d/exim4 restart

That same macro is used for both the single monolithic config file, and the split config file.  It tells Exim that for remote SMTP only, it should rewrite any header that matches the left part of the line with the replacement on the right.  The ${1} on the right matches the * on the left (multiple *’s can be matched with ${1}, ${2}, etc.)

You can supply multiple rules by separating them with colons, such as this,

REMOTE_SMTP_HEADERS_REWRITE = *@hostname.example.net ${1}@example.net : *@hostname ${1}@example.net : *@localhost ${1}@example.net

There are more flags you can provide to the rewrite rules, and you can place rewrites in other locations, but the above will achieve the basic desire of ignoring locally delivered mail, but rewriting all headers on outbound e-mail which match.

Full details of Exim’s rewrite stuff is here.  Details about using Exim4 with Debian can be found in Debian’s Exim4 readme (which you can read online here).

I’m sure there are other ways of achieving this, there’s certainly an option in the debconf config (dc_hide_mailname) which seems hopeful, but it didn’t seem to do anything for me (maybe it only works when you’re using a smart relay?)  Either way, this option does what I wanted, and hey, this is UNIX, there’s always more than one way to skin a cat.

Edit: just had a look at the various Debian package files, and it looks like dc_hide_mailname only works if you’re using a smart relay option for Exim.  If you’re using the full internet host option from Debconf, it never asks you if you want to hide the mailname, and ignores that option when you rebuild the config files.