Exchange 2010 Delivery has failed to these recipients or groups

We transferred a Client's email from a POP3 service to Exchange 2010 and at least one user has ended up with the following auto reply message when sending to distribution groups.
The distribution groups are stored locally on his PC. It was used before the migration.
_____
Delivery has failed to these recipients or groups:
 
Firstname Lastname (user@domain.com)
The e-mail address you entered couldn't be found. Please check the recipient's e-mail address and try to resend the message. If the problem continues, please contact your helpdesk.
_____

Attached is the process that we used to move the mailboxes.  2Converting-email-to-the-exchang.docx
StarfishTechAsked:
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Neil RussellTechnical Development LeadCommented:
You need to delete and recreate the distribution group. It will have a legacy translation that points to the premigration email address. When recreating the distribution group, pick each person from the global address book if internal.

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StarfishTechAuthor Commented:
OK, that makes sense but we are still in the middle of the migration. What if a user hasn't been migrated to the exchange server yet? Should they still be added via the Global address book?

Example - userA is still a pop user but userB is on exchange server now. When recreating the distribution list, should both userA and userB be added to the list in the same fasion even though one is an exchange mailbox and one is a pop mailbox?
Neil RussellTechnical Development LeadCommented:
Are those that are on pop3 accounts using the same domain name? user@domain.com for both exchange and pop3?
StarfishTechAuthor Commented:
Yes, we are just migrating the users from a pop server to an exchange server so we didn't want their email addresses to change. Therefore, userA is user@domain.com on the pop server and on the exchange server
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