This article will point you in the right direction on how to configure SmarterMail to support SDR (Split Domain Routing).
In December 2022, Rackspace mail got hacked and for over a month some of our Exchange clients did not have any emails due to the sheer incompetence of the Rackspace Mail team.
Check out the fiasco here: https://www.jucra.com/whmcs/announcements/112/
The problem we have was that we have many clients set up with hybrid hosting on Rackspace meaning that some were on Rackspace Hosted Exchange and some were on normal Rackspace Mail and as Rackspace's Exchange system was completely borked/hacked/owned, anyone using Rackspace Hosted Exchange basically could not access their email since 2nd of December and even today on 2nd of January, a month later it's still not clear if our clients will get their email history back.
So, for this workaround, you need to have at least the free version of Smartermail which allows 1 domain with up to 10 users. If you have many domains or email accounts, the professional version of SmarterMial is $499 per year for up to 250 accounts.
In this example, we will assume that the client's domain name is mrclient.com and we will assume you have 1 user called bob@ who was on Hosted Exchange but all other users were on normal Rackspace mail.
STEP 1
Install SmarterMail on a compatible windows server.
https://www.smartertools.com/smartermail/downloads#/pills-free
STEP 2
Configure the Windows server firewall to allow smartermail ports (this part is crucial otherwise mails wont come in).
https://portal.smartertools.com/kb/a2838/commonly-used-ports-for-smartermail.aspx
STEP 3
Create an A record on mrclient.com pointing to your SmarterMail server which will be the new mx record for the domain. EG: mx.mrclient.com. Point that domain to your SmarterMail server IP.
STEP 4
In the client DNS, add mx.mrclient.com as an MX record with a TTL of 0 and delete the mx1.emailsrvr.com and mx2.emailsrvr.com. Mail for mrclient.com will now be routed to the SmarterMail server.
STEP 5
Add the domain mrclient.com to SmarterMail. In this example, we want to add a user called bob@ because bob's email was on change and now he cant get it.
STEP 6 - This is the magic!
Set up "external mail routing" for the domain and put in the Rackspace MX record as the "External Host Address" and check the "Deliver Locally if user exists".
What these settings do, is, deliver any email accounts LOCALLY to smartermail but deliver any non-local recipient to Rackspace mail. This is the SDR.
So if bob@ had a Rackspace Exchange account, you would set up bob on Smartermail so that anything for bob@ goes to smartermail, and anything else for any other Rackspace Mail user gets pushed over to the Rackspace Mail system with no additional configuration.
Okay, sure, bob is going to have to use SmarterMail for a while but TBH, it's not a bad interface.
NOTE: This could also work for any externally hosted email account BTW, such as Google, MS365 etc.
And that's basically it!
Yes, you need to make sure your SPF records are updated so bob can send emails from his new account, and probably you need to install some SSL/TLS certs but you get the gist.
