It has been a while and offering this from memory and notes, but this was a pretty straight forward process going from 2008R2 to 2016, I've not tested 2019, but presume the process will be similar if not the same.
Presuming you are using the same domain and sub domain used on the source for the target.
If you use SQL / SQL Express, you'll want to load it on the new VM and patch as needed.
Bring the mailserver services down on the source VM and try a backup of the existing server:
You will need to be in the appropriate directory, e.g., C:\Program Files\ArGo Software Design\ArGoSoft Mail Server .NET>
You will want to migrate the .lic file from the source to the target as well as exporting any certs needed from the source.
Migrate the files you created and retrieved above to the new server.
You will have to install a copy of the MailServer to the new system and though I don't seem to have the restore program in my notes on this, if memory serves, it is something like DataRestore.
You will use the directory that you ported over to the new VM which contained the data from the DataBackup step.
Register the license file you brought from the old machine to the new machine.
Make sure the certs are added as needed and try firing the services up.
No guarantees, but it should work without too many hitches.
Obviously, once transferred and confirmed as successful, the old VM copy of ArgoSoft MailServer must be retired.
Good luck and hopefully I have been both clear enough and haven't missed any steps.