If your Mac Mail client fails to load new messages after the mailbox migration, you'll need to first remove your account, then add it back to establish a connection. Removing the account does not impact messages in your email account.
Remove email account
With the Mac Mail email client open, click on Mail, then Accounts
Select the account to remove, which will have your current email address. Press the minus "-" sign to initiate the removal of the account.
Verify you'd like to remove the email account.
Your account is now removed from the MacMail email client.
To Add Your Email Account
On the Internet Accounts screen, press the plus "+" sign and then the Exchange option.
Enter your account information including name of the account, full email address, and BeachID password.
Select apps you'd like to synchronize.
Select Done and your mail/calendar/contacts will now synchronize.