It's important to remember that AWS blocks the SMTP port for new clients and machines. Before migrating, make sure there are no blockages on the ports.

Andrew.. a follow up question:  Can the Migration Assistant using IMAP/EWS be run multiple times subsequent to the initial migration? 

There will likely be new mail messages received by the old mail server while the new mail server MX records propagate, so I'm wondering if we can run Migration Assistant multiple times to continue migrating new messages until the MX changes are successfully sending mail to the new server.

@jack - Thank you! Great tip!

Hi Gregor!

To do what you ask (i.e. migrate new mails that could arrive on the old server/account after the initial migration), I suggest doing this from the beginning:

1 - use the "MAilbox Migration" tool ONLY AND EXCLUSIVELY to migrate Calendars/Contacts/Tasks/Notes (you do NOT have to migrate the mails!!!!)

2 - use the "Email Retrieval" tool to migrate the mails (possibly DO NOT use POP3, but instead IMAP or the specific tool for the server to migrate), setting the "Retrieval Method" to "Automatic"

The way to do it would be to set the DNS to the new server, make sure all mails arrive.

Start the intial migration 24hrs after changing the DNS to the new server. THen you are sure all servers are updated and no mail will be received on the old server.

And when you do then export the old mailbox from a specific user to a .pst.

Import the .pst via outlook running in cached mode on the users pc making the user transfer the mails.

That way nothing wrong is happening and you always have a working backup PST.

Gregor,

The most likely outcome of running the migration multiple times is that data will be duplicated in the new mailbox. Between runs, the migration utility does not keep track of what data has been previously retrieved. The migration utility does have the option to delete all items before the migration, but using that would be just as effective as running the migration once.

As Gabriele suggested, you could do a migration of the Calendar, Contacts, and Notes using EWS, then set up message retrieval using either EWS or IMAP.

Ultimately, if a more gradual migration is a viable option for you, I would do as Brian suggested and make sure mail is being delivered to the new server before configuring the migration from the old server. Then you don't need to worry about new data coming in during the switchover.