The new version should be able to convert your old mailboxes for you. Yes, you can copy the mailbox structures intact, and probably some other configuration files. I don't have specific instructions, but you should get them from ST or someone who has done it.

As I don't need the latest features, my SM clients are still on the last build of V15 -  Enterprise 15.7.6970. This version has proven to be remarkably stable - installed February 2019; the only maintenance I have needed is: backups, maintaining accounts and passwords and increasing user disk space as necessary. And I restart the dedicated SM server (2012 R2) after hours for O/S updates. Every so often I have to clear a user's browser cache. Not needing MAPI or desktop Outlook means I can stay here for awhile.

A handful of users who aren't happy with Webmail (most are) are using Thunderbird. SM works fine with most smartphone email apps.

yes, there is.

You can find a post about it here somewhere.

But basically its something like this.

Create new domain with an admin user on the new server.

On the new server delete all the files in the domain folder.

Stop the mail services

Copy all files and folders from the old server.

Start the new mail server.

It will convert the users to the new format.

you are also supposed to be able to use the attach domain feature but not tried that.

Find the post before you do this since my memory might have missed a step.

Thank you Paul and Heimir. I found this:

I'm gonna go ahead and do it now. Fingers crossed...

That sounds about right, now that I see it. BTW make sure you see the entries on this forum about a few important issues that were fixed by a custom build and/or look for a new release. 

Thank you guys, I got it all done, everything's working.

In the instructions it says to copy the settings directory, but I didn't have that directory on my old server. So I set up IIS and created a new domain that had the same name as the one on the old server and the same primary admin. Then I copied the domain directory from the old server. It worked. I had to mess a little bit with settings and rDNS and what not... I don't do this sort of thing on a regular basis so it took me some time, but I got it sorted out.

I have to say SmarterMail has served me well over the years. Fingers crossed the new version is stable and works well too.

Nice work!

Thank you! :) 

I do have one more quick question. I didn't set up SMTP TLS. This server is mainly used to send marketing info to our clients and set up appointments, things like that. Nothing secret, nothing that needs protecting. Is there a reason I would need to bother with encrypting SMTP?

Probably because many servers will not accept email if not via SMTP TLS or will at least give it a higher chance of it being SPAM.

At this point it`s just a good practice to use HTTPS / TLS everywhere.