PowerShell Code: Find Users with Email Address From a Specific Domain

Searching Email addresses with a specific domain name suffix Using PowerShell | Office 365 

Find Users with Email Address From a Specific Domain

Get-Mailbox -ResultSize Unlimited | Where-Object {($_.EmailAddresses -like "*@YourDomain.Com*")}

Command may take long to run as it will run on your Tenant completely. 

Use | Export-CSV Command to extract the output to the CSV File.