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I am migrating all my company mailoboxes, that actually are located on an internal IMAP server, to a Office 365 tenant. Since the migrating process whould take a quite long time, I need to share the company domain with the actual server, in order to do the switch mailbox by mailbox. So at the moment not all the O365 mailbox are assigned to our company domain. I need that all the outgoing emails from O365 to our company domain should pass from external because at the moment I get back a message that tells me that the messages cannot be delivered (the internal O365 account is not yet assigned to company domain). Is that possible? How can I do that? I found reading online that maybe I need a "smart host" but I have no idea of what it is. I also asked MS support but I am struggling to have a response.

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Migrating an IMAP mailbox to Office365 is not that hard.

First, you add the domain to the Office365. This is a 3 step process.

Step 1 is adding an MX or TXT record. Given the old situation still works, you use the TXT record to validate the domain.

Step 2 is setting up autodiscover DNS records and altering the MX records. This is the point where you want to abort the installation for the domain.

As the domain was verified in step 1, you can now create mailboxes for all users.

This is the stage where both IMAP and Office365 live side by side, but the MX records determine where new email lands.

You can configure Microsoft Outlook manually using server adress: outlook.office.com or use a tool such as imapsync to get the email from the IMAP account into the Office365 account. You prepare for the sync. Right before this is done, you alter the MX records and perform the installation of the domain and complete step 2. Step 3 means all is setup correctly, and the Office365 environment is live.

You start the sync and while the sync runs, you configure the new accounts for the users. This ensures they get new email while the old email pours in slowly. It is adviced to setup cache for a week if they have slow internet such that their internet won't get filled from all outlook clients downloading the mail. Once all mailboxes are up and running, one by one, set them to 6 months or a year or whatever you want to give them access to.

This allows them to get the older mail by clicking a link in their outlook if they need to get it until the entire migration is done.

NOTE: if the internet is poor, it might be better to create PST backups of the IMAP mail on the clients, and import that after the new mailbox was setup. this ensures that there's only an upload, not a download, but this could lead to problems too. In either case, you have to come to a point where you switch MX records and then you migrate.

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  • I am already syncing about 100 mailboxes with Imapsync, some of these are big (over 70gb). Some are already synchronized others not yet. For the mailboxes already synchronized I want to configure outlook to the 'new' O365 account, but on that mailboxes I cannot write to my other domain mailboxes (those not synchronized) because there are not assigned to the domain (still using old IMAP). So I need that the mail addressed to my domain to be delivered following the 'outside' path. How can I do that?
    – Daniele Goffi
    Jun 9, 2020 at 19:24
  • Get everyone over to the new domain ASAP, so they all use Office365, even if that means they don't have access to their old email. In the background perform the imap sync, and tell users it will take a few days for the old email to come back. Don't get in a scenario where you have old vs new. That is simply not possible, because you cannot migrate without risking of not syncing mails from in-between-migration moment.
    – LPChip
    Jun 9, 2020 at 20:28
  • What you can do is setup the new office365 email using the .onmicrosoft.com alias that is also created so you have the non-working imap and office365 next to eachother. They can use the non-working for archive purposes only though, not send email using it. This is prone to human error though, so only recommended as a very last resort.
    – LPChip
    Jun 9, 2020 at 20:31
  • Unfortunately I cannot do this. I have to change the configuration to each user one-by-one (they are about 100, some of those in other countries) and in the meanwhile they cannot write to other internal domain addresses until I switched every mailbox. This is not suitable, there must be a way to send mails to our domain using MX instead of O365 'internal' route.
    – Daniele Goffi
    Jun 10, 2020 at 6:43
  • The only way to be able to mail to other users that are not yet migrated, is by not setting up the domain yet. Users have 2 mailboxes. 1 in Office 365 using [email protected] account and [email protected] for imap. MX records point to the imap and you setup the office365 account in their outlook first. You wait with the migration until all outlook accounts are setup, so the old situation remains until outlook runs 2 accounts. In Office365, you give each user the alias [email protected], and once everyone is setup, you make it primary.
    – LPChip
    Jun 10, 2020 at 7:21
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It is exactly what LPChip has said above, in addition, i found some articles for your reference: What you need to know about migrating your IMAP mailboxes to Office 365 and Mailflow Co-existence between G Suite and Office 365 during IMAP Migration

Here's a Microsoft official article, i believe it may be helpful to you. Mail flow coexistence between Gmail and Exchange Online during mailbox migration phase

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