parallelbetrieb

wizzurdofodd I did exactly this when I was trialling Grummunio to replace my Exchange 2016 server nearly 2 years ago. However, you will need to put an SMTP MTA that can forward mail to more than one server infront of both your Exchange and Grommunio servers. There are caveats with this approach as it only allows testing/evaluation for email. Calendar, contacts etc will only be usable within the Exchange OR Grommunio ecosystem, as they use different databases that are not shared. So this is really only for testing the email functionality.

There may be other ways of sending incoming emails to both Exchange and Grommunio servers and there are bound to be many other MTA's other than the one I used that can do the same task, but the method I followed is below.

I was already using a program called anti-spam application called Xeams to scan all my email for viruses and spam, and this has an option to forward some or all incoming emails to multiple backend mail servers by either individual email address or whole domains. There is a free community version for upto 5 mailboxes.

As I was already using Xeams it was setup to receive my SMTP email from the internet, scan it for spam and viruses before passing it onto my Exchange 2016 box and the user mailboxes. When I found out about Grommunio all I needed to do was build a test Grommunio server, setup a forwarder (in Xeams) to duplicate the incoming feed to my test Grommunio server allowing me to investigate how Grommunio worked and how I could implement it into my IT system. When I was happy that I could use Grommunio in place of Exchange 2016, all I needed to do was export my mailboxes to PST, create a new cached mail profile in Outlook connected to the users Grommunio account and then import the PST. The cached profile then migrated the users emails etc into Grommunio and the users could then use the Outlook profile connected to Grommunio instead of the one connected to Exchange.

Since incoming emails were still being routed to both the Exchange 2016 and Grommunio servers if any unforeseen issues were found with the Grommunio configuration the user(s) could go back to their Outlook Exchange profile (yes they would have to reprocess and emails that had been processed in the other profile) but the two systems could be reasonably worked alongside each other. Once all issues were worked out in the Grommunio setup the user account could be removed from Exchange 2016 and Xeams would just report 'unknown user' in its logs when it tried to send emails to the migrated users Exchange account that was no longer in place, whilst the migrated users email would arrive in Outlook using the Grommunio profile.

Xeams can also be used as a smart mail host so both Exchange and Grommunio can be pointed to it so it also sends outgoing email as well (scanning it for viruses and spam on the outbound route as well).

Hope the above is of interest.