Sounds like you need to search your recipients in Exchange Online and find the mailboxes that match a certain domain name such as 456.com.
This should be the command that you can use to get that info. First you should Connect to Exchange Online using PowerShell.
Then you can run this command:
- Get-Mailbox | Where {$_.emailaddresses -like "*456.com*"} | FL Name,Alias,EmailAddresses
If you need to export to a file use the command below:
- Get-Mailbox | Where {$_.emailaddresses -like "*456.com*"} | FL Name,Alias,EmailAddresses | Export-Csv c:\path\tofile.csv