Ubuntu – Lightweight local mail transfer agent

There are a few choices to be made when selecting an MTA to install; the first one is whether you want a send-only MTA or you need a fully-fledged mail server.

This Unix.SE post provides a comprehensive list of send-only MTAs available; all of them are already packaged for Ubuntu 10.10. There are a few things you might want to consider when choosing a send-only MTA:

1. whether it can queue e-mails for later delivery in case of a failure: IIRC, only nullmailer can do this.

2. whether it will replace the system mail delivery agent (look for a line Provides: mail-transport-agent in the output of apt-cache show package). If it does, then all mail originating from the system (including reports from cron jobs, popularity statistics, etc.) will be handled by the MTA you install: if you do not set up correct mail aliases for root and other users, the email might end up delivered at the wrong address (typically, some sysadmin at your ISP). Currently, packages nullmailer, esmtp-run (but not esmtp) and ssmtp fall into this category.

3. All send-only MTAs will just forward your email to another server (called "relay host" or "smarthost"; typically it's your ISP's SMTP server) and then let it handle all the details of real Internet mail delivery. You need to figure out whether there are any special requirements for connecting to the ISP server (e.g., some kind of authentication or TLS) and choose an MTA that supports the required features (for instance, ssmtp does not support any kind of authentication or SSL/TLS).

Fully-fledged mail servers include exim, postfix, and sendmail. The post-installation installation script for Debian/Ubuntu will do a great job of producing a working configuration for some common setup (delivery of mail to the local system only; sending email through a "smarthost" -- i.e., a mail server you funnel all your emails through, typically your ISP's SMTP host; internet site with full inbound and outbound connectivity). IMHO, postfix is the easiest to run and configure, but since it's so capable, its configuration manual is way longer than any of the send-only MTAs.

In addition, the real risk with setting up a mail server is that, if you make a mistake in the local configuration, you end up with mail delivered to the wrong place (read: lost) or -worse- with an open-relay system that can be used by spammers (and, trust me, they can find an open relay before you realize your server is one).

My advice would be then to start with a send-only MTA and then upgrade to a full mail server only if you see the need for it (which is not the case if you just want to send emails to your GMail address).

I finally found a detailed set of instructions by Tony Scelfo that actually work. It appears that you have to use transport layer security (TLS) on port 587. I have not gotten SSL SMTP to work.

First run sudo dpkg-reconfigure exim4-config and use these config options:

• General type of mail configuration: mail sent by smarthost; received via SMTP or fetchmail
• System mail name: <your hostname>
• IP-address to listen on for incoming SMTP connections: 127.0.0.1
• Other destinations for which mail is accepted: <your hostname>
• Machines to relay mail for: <leave this blank>
• IP address or host name of the outgoing smarthost: mail.example.com::587
• Hide local mail name in outgoing mail?
  • Yes - all outgoing mail will appear to come from your gmail account
  • No - mail sent with a valid sender name header will keep the sender’s name
• Keep number of DNS-queries minimal (Dial-on-Demand)? No
• Delivery method for local mail: <choose the one you prefer>
• Split configuration file into small files? Yes (you need to edit one of the files next)

Then run sudo vi /etc/exim4/passwd.client and add the following lines for your mail host, and any aliases it has (found through nslookup). Substitute <email address> and <password> with the account you want to route mail through):

mail.example.com:<email address>:<password>
mail.yourhosting.provider:<email address>:<password>

Once you edit the passwd.client file, run sudo update-exim4.conf which will integrate your changes into your Exim4 config.

Run sudo /etc/init.d/exim4 restart and make sure that the service stops and starts properly. If the service is unable to restart, something probably went wrong when you edited the passwd.client file.

If Exim4 restarted, go ahead and run sudo tail -f /var/log/exim4/mainlog to watch the mail logs. In a different window, send an email from your system and make sure that you see a record go by withR=smarthost T=remote_smtp_smarthost H=gmail-smtp-msa.l.google.com ... X=TLS-1.0:RSA_ARCFOUR_MD5:16 in it. The X=TLS means that the mail is being sent with transport layer security which is what you want.