Let's say I have domain.com , domain2.com, domain3.com on same VPS and all of them send mail, but Helo of all messages from all domains is localhost which gets rejected by some mail providers. How to make exim4 set helo to domain name from where it's sending mail. Like if I'm sending mail from domain.com helo should be domain.com. Other question, is this right thing to do?
UPD I changed my mind. I just want to set it to FQDN of box.
The smtp transport has a helo_data setting. By default it is set to $primary_hostname, which is why the first person's advice would work just fine if you wanted to configure the primary_hostname setting to a single, valid hostname. Since you seem to want to make it HELO with a dynamic hostname, you have to add a little logic to figure out which name it should use.
Somewhere in your DATA acl, you could detect those specific domains and set a variable. This is a very simple example (you could enhance by using file or database lookups) :
warn domains = domain1.com : domain2.com : domain3.com
set acl_m_special_dom = mail.$domain
Look in your logs to determine which smtp transport is being used to send this email. Edit that transport and add a helo_data line that examines that acl message variable and adjusts the hostname that exim will use to HELO/EHLO with:
helo_data = ${if eq{$acl_m_special_dom}{} \
{$primary_hostname} \
{$acl_m_special_dom} }
The test checks if the variable is empty. If it's empty, it will use $primary_hostname. But if it is not empty, then it must have been set in the ACL condition above so it uses that hostname in the EHLO. This variable is a per message variable, so it gets cleared inbetween each message (if multiple messages are coming down in one connection).
You must make sure that any hostname you use here is resolvable in public DNS (test using Google's DNS), and that the IP it resolves too also has a reverse DNS. The rDNS does not need to match the hostname you use, but it needs to resolve to something. And if it reverse resolves to something that looks dynamic like a cablemodem or dsl line, it will likely still get blocked or deferred by most major mail providers.
Back to your description, it sounds like you are using the Debian split configuration system. They have a macro that will vary the HELO hostname based on the sender IP address that connects to your exim server. This will not work for you though if your email is sourced from one common server, meaning you need to vary this HELO hostname based on the sender domain name as shown above.
The "proper" way to do this would be to set Exim's "primary host name" to whatever the fully qualified, canonical name of your host is. Few remote mail servers will even begin to care about the domain part in the From: header of the mail, and most don't give the domain part of the envelope-sender a second glance either if it passes such things as SPF validation.
The exact steps for doing this would depend on your installation, but it looks like simply setting Exim's primary_hostname configuration variable to a FQDN that resolves back to your host's public IP address will do nicely.
You may find http://www.exim.org/exim-html-current/doc/html/spec_html/ch07.html to be helpful.
This defaults to the server hostname provided by uname, and can be changed via CLI with the "hostname" command. BSD systems us the hosts file to store the system hostname.
If you want the server hostname to identify differently from the helo name (such as I like to do with a dedicated second IP address, along with some other configurations), you can set $primary_hostname in the exim configuration. Since $helo_data defaults to $primary_hostname, this covers both sending and receiving configurations if $helo_data is left at default.
Knowing this variable, there may be a way to do a lookup table to achieve your per-domain setting change using this setting. I've seen things in configurations such as this:
${if exists{/etc/userdomains} {lsearch;/etc/userdomains} fail}
