Hi,
Thanks for posting your question in the Microsoft Q&A forum.
Based on your description, I would suggest that you use the Import-Csv " location of the CSV file \name.csv" | ForEach-Object { Get-Mailbox -Identity $_.DisplayName | Select-Object DisplayName,PrimarySmtpAddress} cmdlet to get the user's email address.
Make sure your CSV file has a column named "DisplayName" and a column named "PrimarySmtpAddress".
Here are my test over results:
If my reply helps you, please mark him as an answer. Thanks for the support and understanding.