Will mail work this way (+another server)?

Question

Email

Now the organization uses an ancient mailer, compiled by someone once on one of the niks platforms with a screwed web interface (for management).
For a number of reasons, this has ceased to suit, but mail is an important tool for the company. You need to move very smoothly.
Hence the question. If I install another mail server (it doesn’t matter which one, now the question is general) side by side, give it a white IP (by the way, there’s no way without it?) And start a second MX record - in theory the old one will continue to work for itself, and the mail will fall on both the old and the new, right?

Answer 1

try making a shared file storage environment and navigate through it. Or, for example, do synchronization from the old to the new server with rsync - you will have a working server and an up-to-date mailbox database, then change the records in dns and everything will be ok

Answer 2

Mail will arrive in the order in which the addresses will be given by the DNS server. But not on both servers at the same time, as you would like.
The mailer will also work behind NAT if the port is forwarded to it. But this is a crutch solution.

Answer 3

But what to do?