Mx record during migration

We currently have a POP email server and are going to migrate to Office 365 Online Exchange server.  Our domain name points to our POP server provider and I want to make sure we don't lose emails and the transition is seemless.  

Is it possible to allow emails to first come to the Exchange server and if there is no existing email account, then it will propagate to the old server?

What is the best way to make sure we don't lose any emails or that we are transitioning to Exchange without any loss.  How would I setup the MX records to avoid loss?
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Mohammed KhawajaManager - Infrastructure:  Information TechnologyCommented:
Create an internal relay domain and create a send connector to route email for the domain to your POP3 server.  Refer to link below and it works the same for 2013 (the link is for 2010):

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What you can do is configure a catch-all mailbox on o365 until you resolve the missing email addresses after which point you can disable the catch-all allowing for misdirected emails to bounce back.
al4629740Author Commented:
How do I setup a catchall mailbox?
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Unfortunately, after making a suggestion on commonly available options, o365 does not support catch-all.
The mechanism would be to setup a shared mailbox into which you would add aliases that are missing.

You would have to remove the alias once a dedicated mailbox is created, the message tracing available in o365 will help you capture add as needed.  As far as the transition, the first thing you should do is adjust the DNS record reficing the TTL of the MX records to 60 seconds.  Depending on the current TTL, you would need to wait that long after which a transition will be almost instantenpeous.

You should however do the following make your users aware that they may need to check both for a short duration.
Shreedhar EtteCommented:

Please let me know how you are planning to migrate data from your POP email solution to Office 365. Based on that we can plan about cutover of the MX Records.

If your users data is stored in local PST then below will be the migration option:
1. Verify your email domain on office 365.
2. Create all the Users and Groups on Office 365.
3. Point your MX Records to Office 365.
4. Perform Last Send/Receive in outlook client.
5. Create New Outlook Profile for Office 365 mailbox.
6. Import PST to New Outlook Profile.
Will SzymkowskiSenior Solution ArchitectCommented:
Unfortunately if a mailbox is not present it will send an NDR back to the sender stating the account does not exist. What you could do is export the mailbox and import them into your O365 mailbox leaving the other POP mailboxes in tacted for a period of time with an auto reply or out of office message stating the new address. This way you can retrieve any left over email after the export and the senders would know your new address.

Once the migration is completed nuke the pop mailboxes.

Mohammed KhawajaManager - Infrastructure:  Information TechnologyCommented:
I guess the best solution would be to point your MX to your POP3 email and configure that server to forward unresolved mailbox to Office 365.  If there is no mailbox either on POP3 or Office 365 then Office 365 will send an NDR message.
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