MX Records

Im currently doing a migration of several users to O365, Ive changed all the DNS records apart from the MX records, however cannot add users alias's until I complete the DNS record, which I dont want to do until we are ready to migrate.

Can I ignore the MX record warnings and then add the alias, then when ready to migrate change the MX records? From my understanding users who send to my users on O365 external to my account O365 will still use the MX records and the emails will goto the current MX record destination. Is that right? This means they can continue to send and recive emails on the old system until I flick the switch and change the MX records, then all the users and alias's are already setup.
tonelm54Asked:
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Vasil Michev (MVP)Connect With a Mentor Commented:
Yes, you can ignore the warnings. All you need to do is verify the domain and set its purpose to Exchange Online, which you have already done.

Mail will continue flowing to the old system though, which might or might not be a problem :)
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Steven RundleCommented:
I just did an Exchange Online migration.

While in the process, you basically have a hybrid setup. Your on-premise users that you synchronize will become mail contacts to your online exchange organization, and your online users will become mail contacts to your on-premise organization when the mailbox is migrated.

It can get a little  weird when your clients are cached mode and they have outdated OABs or you are missing distribution groups on the online side, and that can be a little bit of a pain as you migrate, but mail should continue to flow between your online and on-premise users without too much trouble. On-premise users will send mail out through your on-premise organization, and online users will send mail out through MS's servers.

Your user accounts will have a couple different attributes that are relevant here. Proxyaddress, which should contain SMTP:<user>@domain.com as well as smtp:<user>@domain,onmicrosoft.com and possibly also smtp:<user>@domain.mail.onmicrosoft.com

External SMTP servers will resolve MX for domain.com and see your on-premise server, and send the mail there. If the the destination user's mailbox has been migrated, they should show up as a mail user and will have a targetAddress attribute that tells your exchange to reroute the mail back out to <user>@domain.onmicrosoft.com.

You just need to have the appropriate connectors set up on each end to handle the mail flow. The Hybrid wizard sets them up for you.
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