How to have different mail servers for different users?

Currently we are running on an older mail service where each client uses IMAP and SMTP to connect for incoming and outgoing mail. This is not Exchanged-based - let's call it "old school" mail server with little options for configuration. It doesn't support more modern push services for mobile devices, and its SPAM filtering and mailbox filtering is poor or non-existent.

I would like to setup a more modern mail server and slowly move my users over to it. This might be Google Apps for Business, but it might also be Office365, we haven't decided yet. In any case we want to move to a service where we have more fine-grained administrative control over SPAM rules, and also server-side filtering and mailbox rules (right now everything is client-side).

How would I go about "splitting" incoming email - so some of it continues to deliver to the old system and those users can continue to use that system unaffected, and the users I move over can have email delivered into their newer, more modern email server?

AFAIK there is only one set of MX records, and none of them delineate anything about users, but it's been quite some time since I setup email services so I may be out of date and/or missing something.

jimmy0x52

Posted 2019-10-17T14:31:20.797

Reputation: 121

If the "old school" mail server supports forwarding rules, it can forward some mail to the new server, until it is totally replaced. At worst, you may set up a frontal server that will split the mails. – harrymc – 2019-10-17T14:54:28.897

Can you elaborate on this "frontal server" concept? – jimmy0x52 – 2019-10-17T15:25:04.497

Presumably it's just a third server that acts as your MX and only forwards mail to either the old service or to the new one. – user1686 – 2019-10-17T15:44:10.590

Answers

If possible, use the new system in your MX records and configure it to forward either specific mailboxes or all "unknown" mailboxes to the old system. That way both systems will benefit from the spam filtering done by the new system.

For example, G Suite allows this:

Doing the opposite – forwarding from old to new – can be problematic because it disguises the original sender system, so the new spam filter will not be nearly as effective. Not to mention that the old system will have to work with nearly twice the amount of messages (incoming + forwarded outgoing) and you will need to change things again if the old system is ever decommissioned.

user1686

Posted 2019-10-17T14:31:20.797

Reputation: 283 655

A complicating factor with that setup is that G Suite is aggressive about calendar invites if they think they own a domain. I tested G Suite without moving users over and once I put in our domain, all invites from users that were also in the Google sphere of influence started having a lot of issues (right now we use iCloud for calendaring with individual user accounts - it's a small company). If there was a way to use G Suite but tell it not to do calendaring, this might be more attractive. – jimmy0x52 – 2019-10-17T15:24:24.393

Well, you can turn off Google Calendar through Google Admin -> Apps -> Core services, but I don't know what it'll do to calendar invites. – user1686 – 2019-10-17T15:43:11.967

I believe that only disables it for users, but I don't think it will keep it from trying to associate them with your GSuite account vs another 3rd party account. I'll have to muss with it and see. – jimmy0x52 – 2019-10-17T15:45:23.373

You have an unlimited number of options, since you control both the servers and the domain. Just to mention some of them:

Establish the new server and forward "old->new" or "new->old" according to their capabilities.
Transfer temporarily the domain to any provider that can split the mail between old and new servers, finally moving the domain to the new server.
Add to the domain "company.com" a sub-domain "new.company.com" for some few test users, diverting the "new" mail to the new server. This will not cause any problems to the other users, whose setup stays exactly as before, and just needs some additions to the domain records..
Buy another domain name which is a variant of the old one, say "companynew.com", for the new server, finally moving "company.com" to it.

One of the solutions, or a combination, may be the most feasible in your setup, but the details depend on your constraints and actual setup.

harrymc

Posted 2019-10-17T14:31:20.797

Reputation: 306 093