We need to migrate from existing e-mail server (Axigen ) to GW12.
The main concern is how to do that as quickly as possible or step by step without disrupting mail traffic and users.

Here is the situation:
In local network there is mentioned Axigen mail server serving aprox 70 users at single location.
Our official domain is hosted outside by Hosting provider where their mail server forwards all mail to our DDNS address where is ported to local mail server. Forwarding is performed discrete on user base. Each user@official.domain is forwarded to user@ddns.domain. where local server taking over.

This setting is harder to maintain because it is necessary to maintain users and aliases on outside server manually. However, this setting has some advantages. First of all, forwarding is happened immediately. If for some reason , local server is unreachable, outside server will keep on trying for two more days.
Second advantage is that we can use anti spam and virus blocking at the providers side.
Both functionality we could'nt achieve with simple global forwarding of all mail to local server.
This comment is digresion from main topic.

Outgoing mail is relayed by local server over our Internet Connection provider's SMPT host.
Local mail is delivered locally.

We maintain local DNS server too.

Most users are using POP service, while IMAP is used by few only. All users are using Thunderbird so far.

All users in the future should use full GW capabilities with regular GW client, mobile clients and so on.

GW12 server is installed and up already. It is tested locally and tested for remote sending. So, only remote receiving is not tested yet, so neither correct domain is set yet.

Present users authentication is performed by Axigen server and LDAP connection to eDir. STARTTLS is used between clients and server.

Our plan is simple to change local DNS record point to new GW server, to change NAT/routing point to new server and set correct domain for the GW server.
We expect that this will be transparent for users while they will continue to use Thunderbird (certificate mismatch will be issued by first connection and first mail sending. But we plan to warn users in advance together with instructions what to do. My phone will ring that morning many times, I am sure). Few IMAP accounts we can prepare in advance by move mail manually to local folders and back to GW server later.

Latest step should be migrate users from Thunderbird to GW client. This need to be perfomed manually at each user seat. Each Thunderbird client need to be switched to IMAP and then mail need to move to GW server.
Here is concern about old mail volume. Many users have their old mail archived in local folders by years. Some of them has quite huge amount of archived mails with heavy attachments. However old mail can be moved gradually later.

So, this is our plan. Does anybody has any suggestion, comment or wish to point some problem we didn't aware of?

Thank you.