What is the best way to migrate a mail server?

Question

SMTP

Hello!
It is necessary to solve one problem of transferring, slowly and very smoothly, the mail server. At the moment, there is a need to connect external clients via smtp and the imap server only works using unencrypted protocols, this is not safe, therefore, you need to somehow connect clients in a new way together with the organization of a new server.

As I see. We are deploying a mailer \ relay on the network, which will collect all mail from the Lotus mailer at 143 and after giving it to the client, send it at 993, respectively, with the client at 465 with the server inside the network at 25. But I don’t know how to organize all this yet, in principle there is ideas, but sorting through all the methods and testing it is a chore for a long time. I am sure there have already been such solutions, therefore, why complicate or invent something! Well, in the end, move completely to a new server, i.e. it should be possible to switch it from the forwarder to a full-fledged mailer. I will also add that letters, or even directories with letters such as incoming, deleted, outgoing, should be on both the new and the old. That is, all letters should be both there and there, this is in case everything goes according to pi ... .. e.
There is
a ProxMox Gateway
Linux (Haven't decided yet which assembly or ready-made solution (Except ekschenzh))
Lotus Domino
Separately about the mailer Lotus works only on ports 25 and 143, but another 1352 is for lotus note.
At this stage, it is necessary to get rid of it, slowly, since a lot of external, internal services are connected to it, I can’t turn it off abruptly, I can’t configure or add something, since it doesn’t work correctly, there are a lot of errors, it was previously attacked encrypted , in general, assembled from the surviving parts and still works.

The diagram is drawn as it is now and added with a green dotted line as I want.

Answer 1

I would act approximately according to the following plan:
1 set up a new mail server on linux, which choice is yours: your own postfix, dovecot, mysql, rouncube, postfixadmin assembly or some kind of ready-made zimbra type combine.
2 on the new server would create boxes the same as on the old server.
3 in advance, using the imapsync utility or analogues, I would upload the main part of the mail to the new server from the old server.
4 on D-Day (for example, on weekends) I would announce those works, switch incoming mail to the new server, and add the same imapsync delta of letters that came after the previous upload to the new server.
5 is ready - the old server is no longer in business.