Setting up a mail server in 'the cloud' is notoriously difficult, mainly because many cloud-based IP addresses are already blacklisted, and also because you don't have control over reverse DNS in many cases. One way to mitigate these factors is to use Postini for outbound email from your cloud-based mail server. I've experimented with this in a test environment, with fairly good results. Here are some notes I jotted down. I am indebted to Jeff Roberts for his help with the postfix setup.
Let's assume you have a domain called mydomain.com and you want to set up a mail server in EC2 (could be any cloud provider) so that you can both send and receive email. I will also assume that you'll use Postini -- which currently costs $11/year/user. In my setup, I am mostly interested in sending mail out and less in receiving mail, so I only have one user: email@example.com.
I deployed a c1.medium EC2 instance running Ubuntu 9.04 and I also gave it an Elastic IP. The host name of my mail server is mail01.
I then installed postfix via 'apt-get install postfix'.
For starters, I configured postfix to receive mail for mydomain.com, and to restrict the set of IP addresses that are allowed to use it as a relay. The relevant lines in /etc/postfix/main.cf are:
myhostname = mail01
myorigin = /etc/mailname
mydestination = mydomain.com, mail01, localhost.localdomain, localhost
mynetworks = cidr:/etc/postfix/allowed_relays, 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
The contents of /etc/postfix/allowed_relays are similar to:
These IPs/names are application servers that I configured to use mail01 as their outgoing mail server, so they need to be allowed to send mail out.
When you sign up for Postini, you can specify that the user you created is of admin type. When you then log in to https://login.postini.com/ as that user and specify that you want to go to the System Administration area, you'll be directed to a URL such as http://ac-s9.postini.com. Make a note of the number after the s, in this case 9. It will dictate which outbound Postini mail servers you will need to use, per these instructions. For s9 (which was my case), the CIDR range of the outbound Postini mail servers is 184.108.40.206/22.
Within the Postini System Administration interface, I had to edit the settings for Inbound Servers and Outbound Servers.
For Inbound Servers, I edited the Delivery Mgr tab and specified the Elastic IP of mail01 as the Email Server IP address.
For Outbound Servers, I cliked on 'Add Outbound Email Server' and I specified a range whose beginning and end are the Elastic IP of mail01. For Reinjection Hosts, I also specified the Elastic IP of mail01.
The Reinjection Host setup is necessary for using Postini as an outbound mail service. See the Postini Outbound Services Configuration Guide (PDF) for details on how to do this depending on the mail server software you use.
More Postfix setup
Now we are ready to configure postfix to use Postini as its own outbound relay. I just added one line to /etc/postfix/main.cf:
relayhost = outbounds9.obsmtp.com
(this will be different, depending on the number you get when logging into Postini; in my case the number is 9)
I also had to allow the CIDR range of the Postini mail server to use mail01 as a relay (they need to reinject the message into mail01, hence the need to specify mail01's external Elastic IP as the Reinjection Host IP above). I edited /etc/postfix/allowed_relays and added the line:
220.127.116.11/22 postini ac-s9
To use Postini as an incoming mail server service, you need to configure mydomain.com to use Google Apps. Then you'll be able to specify the Postini mail servers that you'll use as MX records for mydomain.com.
To further improve the chances that email you send out from the cloud will actually reach its intended recipients, it's a good idea to set up DKIM and SPF. For DKIM, see my blog post on "DKIM setup with postfix and OpenDKIM".
This is actually fairly easy to do. You basically need to add a DNS record which details the IP addresses of the mail servers which are allowed to send mail for mydomain.com. You can use an SPF setup wizard for this, and an SPF record testing tool to see if your setup is correct. In my case, I added a TXT record with the contents:
v=spf1 ip4:18.104.22.168/20 ip4:22.214.171.124/20 ip4:126.96.36.199/22 ip4:A.B.C.D -all
where the first 3 subnets are the 3 possible CIDR ranges for Postini servers, and the last IP which I denoted with A.B.C.D is mail01's Elastic IP address.
At this point, you should be able to both send email out through mail01 (from the servers you allowed to do so) and then through Postini, and also received email for the users you created within mydomain.com. If you also have DKIM and SPF setup, you're in a pretty good shape in terms of email deliverability situation. Still, sending email reliably is one of the hardest things to do in today's Internet, so good luck!