Best choice for echange to O365 mail migration

[Erik Waterhouse]

Currently my company's email is outsourced to a third party host, but the powers that be have decided to finally move everything onto O365.

I need to migrate around 50 users mail boxes (and everything that comes with them) to the company's O365 account. I was thinking of going for a cutover migration but as all user accounts already exist on O365 and that can cause the migration to fail (as it wants to make new users) I'm unsure what the best option is.

As it stands the O365 accounts are used mainly for sharepoint access, and I'm concerned that if the users were deleted and then remade through the migration that It'd mean all users would need to be reassigned their access rights to hundreds of sites and folders.

Does anyone have any advice?

[#2]

The options will depend on what exactly is possible on the third-party email system, so best check with them directly. Without knowing the specifics there, we can only suggest using PST import/export and third-party tools, which are probably not the ideal solution...

[#3]

You mention your users are already in Office 365, but: Are they using Exchange Online Mailbox? If the answer is No, then you can still do a cutover migration since you can enable/disable the EXO part of each user license as you need

[#4]

Like others have said, the options and what will be migrated will vary based on your host and what route you take for migrating.  Another option I would at least take a look at is BitTitan (https://www.bittitan.com/migrationwiz/mailboxes/).  I've used them for several migrations in the past, going from email connecting over IMAP -> Office 365.  The also support Exchange -> Office 365 migrations and make it nice and simple/straightforward to perform the migration...especially if you do already have all your user accounts created in Office 365 already.